OSPF is enabled on router A. You execute the following command on router A and receive
the accompanying output:
RouterA#ping 224.0.0.5 repeat 1
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 224.0.0.5, timeout is 2 seconds:
Reply to request 0 from 10.100.100.1, 4 ms
10.100.100.1 is the IP address of a loopback interface on router A. What can you conclude
about router A? Select the best response.

A.
Only the router A loopback interface is participating in the OSPF routing process.

B.
None of the router A interfaces are participating in the OSPF routing process.

C.
Router A is using the loopback interface IP addressas its OSPF router ID.

D.
Router A does not have any reachable OSPF neighbors.

Explanation:
The 224.0.0.5 address is the multicast for OSPF routers. Therefore when you ping to this
address all OSPF routers will reply but from the output we learn that only the local router
responded -> Router A does not have any reachable OSPF neighbors.