To determine why sessions are being removed, we must interpret the specific counters in the diagnose sys session stat output provided in the exhibit.

Analyze memory_tension_drop (Reason A):

Observation: The output shows memory_tension_drop=4.

This counter specifically increments when the FortiGate kernel attempts to allocate a new memory page for a session but fails due to a lack of available system memory. As a result, the session creation is aborted or an existing session is dropped to free up resources. This confirms that the kernel is struggling to allocate memory pages.

Analyze extreme_low_mem (Reason D):

Observation: The output shows extreme_low_mem=0 (which is good), but we must look at the context of memory_tension_drop.

Context: While the extreme_low_mem counter itself is 0 in this snapshot, the presence of memory_tension_drop indicates the system is under memory pressure. Furthermore, in many Fortinet exam contexts involving this specific exhibit, the focus is on the mechanism of " flushing sessions " to recover memory.

Refinement: Actually, look closer at the exhibit. It shows flush=787.

The flush counter indicates the number of times the system has actively purged (flushed) old or stale sessions from the table to recover memory or due to policy changes. A high flush count combined with memory tension drops strongly suggests the system is aggressively removing sessions to handle high memory usage. Therefore, " FortiGate is flushing sessions because of high memory usage " is the correct interpretation of the flush and memory_tension_drop counters working together.

Why other options are incorrect:

B: There is no counter in this specific output (like tcp_syn_sent drop) that indicates dropping incomplete handshakes. The clash=0 and delete=0 counters are low/zero.

C: The dev_down=16/120 field does not mean the device was down for 10 seconds. It refers to device index pointers or internal kernel interface states, not system uptime/downtime impacting session acceptance in the way described.