The suggested answer is correct.
OSPF uses multicast to communicate routing updates.
Reason: OSPF leverages multicast addressing to efficiently disseminate routing updates to other routers within the same OSPF area. This targeted approach avoids unnecessary network congestion by limiting the distribution of updates only to interested OSPF neighbors.
OSPF uses two multicast addresses:

224.0.0.5: All OSPF routers
224.0.0.6: All OSPF designated routers (DRs)

Reasons for not choosing the other options:

A. Unicast: While OSPF may use unicast for certain functions like establishing adjacencies in some scenarios (e.g., point-to-point links), it primarily relies on multicast for routine routing updates. Unicast is a one-to-one communication, which is not efficient for sending the same routing information to multiple neighbors.
B. Anycast: Anycast involves sending traffic to the "nearest" server in a group, which is not how OSPF routing updates are disseminated.
D. Broadcast: Broadcast sends traffic to every device on the network. OSPF avoids broadcasts because they are inefficient and can overwhelm network resources. Multicast is more efficient because it only sends traffic to devices that have subscribed to a particular multicast group.