The error 'no proposal chosen' is often due to mismatched proxy IDs in Phase 2.
Why this answer
The 'no proposal chosen' error during Phase 2, despite Phase 1 being up, indicates a mismatch in the IPsec security association (SA) parameters. Since the remote FortiGate has multiple Phase 2 selectors configured, the most likely cause is that the local and remote proxy IDs (local and remote subnets) do not match any of the configured selectors. Phase 2 negotiation uses these proxy IDs to define which traffic should be encrypted; if they don't align, the IKE SA cannot be established.
Exam trap
The trap here is that candidates often confuse Phase 1 and Phase 2 failures, assuming any 'no proposal chosen' error is due to Phase 1 misconfigurations like PSK or certificates, when in fact Phase 1 is already up, isolating the issue to Phase 2 proxy ID mismatches.
How to eliminate wrong answers
Option B is wrong because mismatched pre-shared keys would cause Phase 1 to fail, not Phase 2; Phase 1 is already up in this scenario. Option C is wrong because aggressive Dead Peer Detection (DPD) settings can cause tunnels to flap (go up and down), but they do not produce a 'no proposal chosen' error; that error is specific to Phase 2 proposal mismatches. Option D is wrong because certificate validation failure would also prevent Phase 1 from completing, as certificates are used during IKE authentication in Phase 1, not Phase 2.