The Reverse Path Forwarding (RPF) check is run on the first sent packet of any new session to ensure that the packet arrives on a legitimate interface. This check protects the network from IP spoofing attacks by verifying that a return route exists from the receiving interface back to the source IP address. If the route is invalid or not found, the packet is discarded. Options B and C are incorrect because RPF checks are performed on the first sent packet, not the reply packet.
References:
* FortiOS 7.4.1 Administration Guide: Reverse Path Forwarding (RPF) Check