which type of ARP related problem can force the use of Unicast Mode (Pivot) configuration due to incompatibility on some adjacent routers and switches?

In a R75 ClusterXL Load Sharing configuration, which type of ARP related problem can force the use of Unicast Mode (Pivot) configuration due to incompatibility on some adjacent routers and switches?

A.
Multicast MAC address response to a Unicast IP request

B.
Unicast MAC address response to a Multicast IP request

C.
Multicast MAC address response to a RARP request

D.
MGCP MAC address response to a Multicast IP request