The correct answers are A and B.

The study guide says:

“If NAT-T is enabled, and there is a FortiGate located in the middle that is running NAT, the sniffer command must use a different filter. In this case, IKE traffic uses UDP port 500, but switches to UDP port 4500 during the tunnel negotiation. Additionally, ESP traffic is encapsulated inside the UDP 4500 channel.”

It also says:

“In some networks, UDP is blocked by firewalls or ISPs. In those cases, you can configure your VPN tunnel to use IKE over TCP in the phase 1 configuration. The default IKE TCP port is 443…”

And the study guide gives the correct capture examples:

No NAT: host < remote-gw > and udp port 500

With NAT and NAT-T: host < remote-gw > and (udp port 500 or udp port 4500)

So:

B is correct because with NAT Traversal enabled, the tunnel may no longer be using only UDP 500. It can move to UDP 4500, so the current filter may miss the traffic.

A is correct because the filter may need to be expanded to include UDP 4500 for NAT-T, or TCP 443 when IKE over TCP is used.

Why the other options are wrong:

C is wrong because restricting the filter to the remote peer IP can make the capture more precise, but it is not required for the sniffer to display output. The problem here is the port/protocol choice, not the lack of a host filter. The study guide examples use host filtering as an aid, not as a requirement.

D is wrong because diagnose debug enable is used to enable real-time debug output for applications, but it does not suppress or invalidate sniffer output. Sniffer capture is a separate command path. Fortinet documentation separately documents diagnose sniffer packet ... for packet capture and diagnose debug enable for debug features.

So the verified answers are: A, B.