Based on the debug flow output, what is the reason the packet is dropped?
The message 'no matching policy' clearly states this.
Why this answer
The debug flow output indicates that the packet was dropped because no firewall policy matched the traffic. In FortiGate, even if a valid route exists, the packet must be evaluated against firewall policies; if no policy permits the traffic based on source, destination, service, and interface, the packet is silently dropped. The debug flow will show a message like 'no matching policy' or 'deny by policy' in such cases.
Exam trap
The trap here is that candidates often assume a packet drop is due to a missing route when the debug flow shows a policy drop, because they overlook that FortiGate processes routing before policies and the debug flow output explicitly indicates the stage where the drop occurred.
How to eliminate wrong answers
Option A is wrong because a missing route would cause a different debug flow message, such as 'no route to destination' or 'route lookup failed', and the packet would be dropped at the routing stage, not at the firewall policy stage. Option C is wrong because an invalid source IP address (e.g., RFC 1918 on a public interface) would typically be dropped by antispoofing checks or a specific firewall policy, not by a generic 'no matching policy' message; the debug flow would show 'invalid source' or 'reverse path check failed'. Option D is wrong because a full session table would cause a 'session table full' or 'no session available' message in the debug flow, and the drop would occur during session creation, not during policy lookup.