逆にRBはダウンしたネットワークと直接接続しており、ルーティングテーブルには直接接続ルートを保持しているためqueryを送信する必要はないのではないか、とも思いました。

ここが誤認の元でしょうか。
設問では

「172.16.1.0/24」のネットワークがダウンした場合の動作はどれか。

とあり、ダウンすることで RB の直接接続ルートがなくなるから「他に誰か 172.16.1.0/24 NW への到達性がある人いますかー」って RB がきくわけですね。RA は RB から学習していたのが RB から広報されなくなるので「誰も教えてくれなくなったなぁ」って思って経路情報を消すだけなのです。（RB からの Query に「俺は到達できないぞ」って Reply は返しますが）

それらしいトポロジ作って RB で 172.16.1.0/24 をダウンさせた時の RA/RB で取得した debug eigrp fsm と debug eigrp packets terse です。

RA#
*Oct 19 11:12:15.637: EIGRP: Received QUERY on Gi0/0 - paklen 44 nbr 192.168.2.2 ←ここで Query を Receive しているが、Sendはしていない
*Oct 19 11:12:15.637:   AS 1, Flags 0x0:(NULL), Seq 23/0 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
*Oct 19 11:12:15.638: EIGRP: Enqueueing ACK on Gi0/0 - paklen 0 nbr 192.168.2.2 tid 0
*Oct 19 11:12:15.638:   Ack seq 23 iidbQ un/rely 0/0 peerQ un/rely 1/0
*Oct 19 11:12:15.639: DUAL: AS(1) rcvquery: 172.16.1.0/24 via 192.168.2.2 from 192.168.2.2 metric 72057594037927935/72057594037927935, RD is 3072 for tid 0
*Oct 19 11:12:15.639: EIGRP-IPv4(1): Find FS for dest 172.16.1.0/24. FD is 3072, RD is 3072 on tid 0
*Oct 19 11:12:15.640: EIGRP-IPv4(1): 	192.168.2.2 metric 72057594037927935/72057594037927935 not found Dmin is 72057594037927935
*Oct 19 11:12:15.640: DUAL: AS(1) Peer total 1 stub 0 template 1 for tid 0
*Oct 19 11:12:15.641: DUAL: AS(1) Dest 172.16.1.0/24 (Split Horizon) not entering active state for tid 0.
*Oct 19 11:12:15.641: DUAL: AS(1) Send REPLY(r1/n1) about 172.16.1.0/24 to 192.168.2.2 for tid 0
*Oct 19 11:12:15.643: EIGRP: Sending ACK on Gi0/0 - paklen 0 nbr 192.168.2.2 tid 0
*Oct 19 11:12:15.643:   AS 1, Flags 0x0:(NULL), Seq 0/23 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/0
*Oct 19 11:12:15.651: EIGRP: Enqueueing REPLY on Gi0/0 - paklen 0 nbr 192.168.2.2 tid 0 iidbQ un/rely 0/1 peerQ un/rely 0/0 serno 18-18
*Oct 19 11:12:15.653: EIGRP: Requeued unicast on GigabitEthernet0/0
*Oct 19 11:12:15.656: EIGRP: Sending REPLY on Gi0/0 - paklen 44 nbr 192.168.2.2 tid 0
*Oct 19 11:12:15.656:   AS 1, Flags 0x0:(NULL), Seq 12/23 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1 serno 18-18
*Oct 19 11:12:15.663: EIGRP: Received ACK on Gi0/0 - paklen 0 nbr 192.168.2.2
*Oct 19 11:12:15.664:   AS 1, Flags 0x0:(NULL), Seq 0/12 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
*Oct 19 11:12:15.665: DUAL: AS(1) Removing dest 172.16.1.0/24, nexthop 192.168.2.2
*Oct 19 11:12:15.665: DUAL: AS(1) No routes.  Flushing dest 172.16.1.0/24

RB#
*Oct 19 11:12:15.319: EIGRP-IPv4(1): rcvupdate: 172.16.1.0/24 via Connected metric 72057594037927935/72057594037927935 on tid 0
*Oct 19 11:12:15.319: EIGRP-IPv4(1): Find FS for dest 172.16.1.0/24. FD is 512, RD is 512 on tid 0 ←フィーじぶるさくせさがいない
*Oct 19 11:12:15.320: EIGRP-IPv4(1): 	0.0.0.0 metric 72057594037927935/72057594037927935 not found Dmin is 72057594037927935
*Oct 19 11:12:15.320: DUAL: AS(1) Peer total 2 stub 0 template 1 for tid 0
*Oct 19 11:12:15.321: DUAL: AS(1) Dest 172.16.1.0/24 entering active state for tid 0. ← 172.16.1.0/24 を Active にする
*Oct 19 11:12:15.321: EIGRP-IPv4(1): Set reply-status table. Count is 1.
*Oct 19 11:12:15.321: EIGRP-IPv4(1): Not doing split horizon. Query stub suppressed
*Oct 19 11:12:15.331: EIGRP: Enqueueing QUERY on Gi0/0 - paklen 0 tid 0 iidbQ un/rely 0/1 serno 16-16
*Oct 19 11:12:15.333: EIGRP: Sending QUERY on Gi0/0 - paklen 44 tid 0 ← Query 送信
*Oct 19 11:12:15.334:   AS 1, Flags 0x0:(NULL), Seq 23/0 interfaceQ 0/0 iidbQ un/rely 0/0 serno 16-16
*Oct 19 11:12:15.335: EIGRP: Enqueueing QUERY on Gi0/1 - paklen 0 tid 0 iidbQ un/rely 0/1 serno 16-16
*Oct 19 11:12:15.341: EIGRP: Received ACK on Gi0/0 - paklen 0 nbr 192.168.2.1
*Oct 19 11:12:15.342:   AS 1, Flags 0x0:(NULL), Seq 0/23 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
*Oct 19 11:12:15.342: EIGRP: GigabitEthernet0/0 multicast flow blocking cleared
*Oct 19 11:12:15.355: EIGRP: Received REPLY on Gi0/0 - paklen 44 nbr 192.168.2.1
*Oct 19 11:12:15.355:   AS 1, Flags 0x0:(NULL), Seq 12/23 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
*Oct 19 11:12:15.356: EIGRP: Enqueueing ACK on Gi0/0 - paklen 0 nbr 192.168.2.1 tid 0
*Oct 19 11:12:15.356:   Ack seq 12 iidbQ un/rely 0/0 peerQ un/rely 1/0
*Oct 19 11:12:15.357: EIGRP-IPv4(1): dest(172.16.1.0/24) active
*Oct 19 11:12:15.357: EIGRP-IPv4(1): rcvreply: 172.16.1.0/24 via 192.168.2.1 metric 72057594037927935/72057594037927935 for tid 0
*Oct 19 11:12:15.357: EIGRP-IPv4(1): reply count is 1
*Oct 19 11:12:15.358: DUAL: AS(1) Clearing handle 0, count now 0
*Oct 19 11:12:15.358: DUAL: AS(1) Freeing reply status table
*Oct 19 11:12:15.358: EIGRP-IPv4(1): Find FS for dest 172.16.1.0/24. FD is 72057594037927935, RD is 72057594037927935 on tid 0found
*Oct 19 11:12:15.359: DUAL: AS(1) Removing dest 172.16.1.0/24, nexthop 0.0.0.0
*Oct 19 11:12:15.359: DUAL: AS(1) Removing dest 172.16.1.0/24, nexthop 192.168.2.1
*Oct 19 11:12:15.359: DUAL: AS(1) Send update about 172.16.1.0/24. Reason: rt net gone on tid 0
*Oct 19 11:12:15.359: DUAL: AS(1) Send update about 172.16.1.0/24. Reason: lost if on tid 0
*Oct 19 11:12:15.361: EIGRP: Sending ACK on Gi0/0 - paklen 0 nbr 192.168.2.1 tid 0
*Oct 19 11:12:15.361:   AS 1, Flags 0x0:(NULL), Seq 0/12 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 1/0
*Oct 19 11:12:15.369: EIGRP: Enqueueing UPDATE on Gi0/1 - paklen 0 tid 0 iidbQ un/rely 0/1 serno 17-17
*Oct 19 11:12:15.370: EIGRP: Enqueueing UPDATE on Gi0/1 - paklen 44 nbr 192.168.3.2 tid 0 iidbQ un/rely 0/0 peerQ un/rely 0/0 serno 17-17
*Oct 19 11:12:15.372: EIGRP: Sending UPDATE on Gi0/1 - paklen 44 tid 0
*Oct 19 11:12:15.372:   AS 1, Flags 0x0:(NULL), Seq 25/0 interfaceQ 0/0 iidbQ un/rely 0/0 serno 17-17
*Oct 19 11:12:15.375: EIGRP: Received ACK on Gi0/1 - paklen 0 nbr 192.168.3.2
*Oct 19 11:12:15.376:   AS 1, Flags 0x0:(NULL), Seq 0/25 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/1
*Oct 19 11:12:15.376: EIGRP: GigabitEthernet0/1 multicast flow blocking cleared
*Oct 19 11:12:15.376: DUAL: AS(1) No routes.  Flushing dest 172.16.1.0/24
*Oct 19 11:12:17.313: %LINK-5-CHANGED: Interface Loopback0, changed state to administratively down
*Oct 19 11:12:18.313: %LINEPROTO-5-UPDOWN: Line protocol on Interface Loopback0, changed state to down