FortiGate is unable to establish a tunnel with the FortiSwitch device it is supposed to...

The issue described pertains to the establishment of a tunnel (likely a CAPWAP tunnel for management purposes between FortiGate and FortiSwitch). Based on typical error analysis in tunnel setup scenarios:

The CAPWAP tunnel failed to come up due to a mismatch in time (Option C): This answer is plausible because time synchronization is crucial for security protocols that underpin tunnel establishments, such as DTLS (Datagram Transport Layer Security) used within CAPWAP tunnels. If the clocks on FortiGate and FortiSwitch are significantly out of sync, the security handshake (which can include timestamp validation) could fail, preventing the tunnel from coming up.

References:

Fortinet's technical documentation typically outlines the importance of time synchronization for secure communications. In CAPWAP/DLTS scenarios, precise time matching is crucial to ensure that the cryptographic parameters align correctly during the handshake process.