The correct answer is D .
The study guide explains that IKEv2 has two initial exchanges:
* IKE_SA_INIT
* IKE_AUTH
and then later exchanges such as:
* CREATE_CHILD_SA
It also states the roles of those exchanges:
* IKE_SA_INIT negotiates the security settings for IKE traffic
* IKE_AUTH performs mutual authentication and sets up the piggyback child SA
* CREATE_CHILD_SA creates a new child SA or rekeys an existing child SA Most importantly, the study guide explicitly says:
"By IKEv2 design, no Diffie-Hellman public key is exchanged during an IKE_AUTH exchange.
Consequently, any phase 2 Diffie-Hellman group configuration mismatch between FortiGate and the peer is experienced only during the first rekey (CREATE_CHILD_SA exchange) of the child SA created during IKE_AUTH." This proves the key idea behind the question: an IKEv2 tunnel can come up successfully first, then fail later during a CREATE_CHILD_SA rekey/renegotiation event because of a phase 2 mismatch. Among the provided options, the matching later-stage cause is mismatched quick-mode selectors during CREATE_CHILD_SA .
Why the other options are wrong:
* A is wrong because if the proposal mismatch were in the initial negotiation path, the tunnel would fail during establishment, not after it was already up. The study guide places initial tunnel establishment in IKE_SA_INIT and IKE_AUTH
* B is wrong because a mismatch in IKE_SA_INIT affects the initial establishment stage, not a tunnel that was already brought up successfully
* C is wrong because a pre-shared key mismatch is part of authentication during IKE_AUTH , so the tunnel would not come up successfully in the first place

References:
Fortinet Technical Note: RPF Default Configuration and Routing Table Matching FortiGate Administration Guide: Routing and Asymmetric Routing Controls Community Knowledgebase: Route Lookups and RPF Enforcement on FortiOS

The correct answers are A and C .
The study guide describes this exact asymmetric ICMP scenario. It states:
"The server sends an echo request to the PC through port2 of the local router, effectively bypassing FortiGate. When it receives the echo request, the PC responds with an echo reply through its default gateway, 10.1.0.2, which is port1 on FortiGate. Because there is no existing session, the echo reply is dropped. All subsequent echo replies are blocked." That means the current problem exists because:
* the ICMP request bypasses FortiGate
* the ICMP reply goes through FortiGate
* FortiGate has no matching session , so it drops the reply
The study guide then shows the exact corrective option:
"Allowing asymmetric routing:"
config system settings
set asymroute enable
end
It further explains:
"After the packet passes through the FortiGate CPU, FortiGate forwards the packet using the FIB, even though there are no session matches. FortiGate forwards all subsequent echo replies using the FIB." So A is correct.
The other valid fix is to make the traffic symmetric by changing the laptop's default gateway so the reply no longer goes through FortiGate. In the exhibit, the alternate gateway is 10.1.0.254 , which is the local router on the same subnet. If the laptop uses 10.1.0.254 instead of 10.1.0.2, the ICMP echo reply follows the same bypass path as the echo request, so the server receives it without involving FortiGate session validation. This makes C correct.
Why the other options are wrong:
* B is wrong because this is not an RPF problem. The study guide explains RPF as a reverse path lookup used to validate whether a packet arrived on a legitimate interface, mainly for spoofing protection. The issue in this scenario is a missing session due to asymmetric routing , not a strict-versus-feasible RPF failure
* D is wrong because FortiGate already has the specific route 10.4.0.0/24 through port3 in the routing table shown in the exhibit, so adding a default static route to port3 is unnecessary and not the reason the echo reply is being dropped So the verified answers are: A, C .