The primary purpose of configuring a main link between the devices is to synchronize session information so that if one unit fails, the other can continue processing traffic without dropping active sessions.

A. To have both sessions and configuration synchronization in layer 2.

This is incorrect because FGSP is used for session synchronization, not configuration synchronization.

B. To load balance both sessions and configuration synchronization between layer 2 and 3.

FGSP does not perform load balancing and is not used for configuration synchronization.

C. To have only configuration synchronization in layer 3.

The main link is not used solely for configuration synchronization.

D. To have both sessions and configuration synchronization in layer 3.

The main link in an FGSP setup is indeed used to synchronize session information across the devices, and it operates at layer 3 since it uses IP addresses to establish the peering.