References:
FortiOS Admin Guide: OSPF, Debug Outputs

To capture encrypted IPsec phase 2 (ESP) traffic between two FortiGate devices, the correct protocol filter to use is ip proto 50. According to the Fortinet official sniffing and debugging documentation, ESP (Encapsulating Security Payload) is used for encrypted phase 2 payload transfer and always uses IP protocol number 50. Running the command diagnose sniffer packet any 'ip proto 50' captures only ESP packets, which represent the encrypted traffic-whether originating or transiting the device.
If there is no NAT device between FortiGates, ESP is not encapsulated in UDP (thus not on UDP port 4500; if NAT-T were required, packets would be UDP-encapsulated, but the scenario explicitly says NAT is not in use). UDP port 500 is for IKE control (negotiation) traffic, and AH (Authentication Header, ip proto 51) is not used for encryption in standard IPsec phase 2 with ESP.
This matches the official CLI reference from Fortinet for VPN and traffic analysis.
**
References:
FortiOS CLI Reference: diagnose sniffer packet, ESP, IP Protocol Numbers FortiGate VPN Administration Guide: Traffic Capture and Analysis of IPsec Traffic

The correct answers are A and B.
The Network Security Support Engineer 7.6 Study Guide states under OSPF Troubleshooting:
"Follow these steps to troubleshoot an OSPF problem between two peers:
Check that the local router can reach the remote peer. IP addresses must be in the same subnet and have the same subnet mask.
Ensure that IP protocol 89 is not blocked.
Hello and dead intervals must match.
The OSPF router ID for each peer must be unique. Duplicate router IDs are not allowed.
Do the MTUs match?
If authentication is enabled, the type and password must match on both sides."** The same page also summarizes the troubleshooting tips as:
"Do peers have an IP address within the same subnet?"
"Is IP protocol 89 blocked?"
This directly confirms:
A is correct
B is correct
Why the other options are wrong:
C is wrong because TCP port 179 is used by BGP, not OSPF. OSPF uses IP protocol 89, as the study guide explicitly notes D is wrong because the study guide's OSPF adjacency troubleshooting checklist does not require an active static route to the peer. OSPF neighbors form adjacency on directly reachable networks, and the guide instead focuses on same subnet, protocol 89, intervals, router ID, MTU, and authentication matching So the verified answers are: A, B.

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.
Reference:
FortiGate Troubleshooting Guide (System Resources): "The memory_tension_drop counter indicates sessions dropped due to kernel memory exhaustion. The flush counter indicates sessions removed to free up table space."