Answer is D.
From the Fortinet Training Institute NSE4 Infrastructure_7.0_Study_Guide page 222
"If IPsec renegotiation takes too much time, then FortiGate might drop interesting traffic because of the absence of active SAs. To Prevent this, you can enable Auto-Negotiate. When you do this, FortiGate not only negotiates new SAs BEFORE the current SAs expire, but it also starts using the new SAs right away..
Another benefit of enabling Auto-Negotiate is that the tunnel comes up and stays up automatically., even when there is no interesting traffic.?

D is for auto keep alive. The answer for auto-negotiate should be B