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?

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 address as 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.