The answer is D.
Page 113: When peering over FGSP, by default, the FortiGate devices or FGCP clusters, share information over layer 3 between the interfaces that are configured with peer IP addresses. You can also specify the interfaces used to synchronize session in layer 2 instead of layer 3 using the "session-sync-dev" setting. When a session synchronization interface is configured and FGSP peers are directly connected on this interface, then session synchronization is done over layer 2, only falling back to layer 3 if the session synchronization interface becomes unavailable.