How can RPF checking be disabled on a FortiGate device?

Disabling RPF (Reverse Path Forwarding) checking on a FortiGate device is achieved through the configuration of the interface settings, specifically by disabling the src-check feature. RPF checking is a security mechanism used to prevent IP address spoofing by ensuring that incoming packets are received on the correct interfaces based on the routing table. If the src-check is disabled at the interface level, this check will no longer be enforced, allowing traffic to flow without the verification of the source address.

The other options do not effectively disable RPF checking. Adjusting the bandwidth of the interface does not relate to the RPF functionality but rather impacts the performance and throughput of the interface. Changing the interface mode could lead to other configuration issues or impact functionality but does not specifically target RPF checks. Resetting the FortiGate to factory defaults would revert the device to its initial state, but it is not a practical or necessary method for disabling RPF checks, as it involves losing all configuration settings, not just those related to RPF. Thus, disabling src-check on the interface level settings is the correct method for achieving the desired outcome.