(Which default port must be allowed by firewalls for the key exchange of the IPsec...

IPsec’s initial key exchange is commonly performed using IKE (Internet Key Exchange), which negotiates Security Associations (SAs), authenticates peers, and establishes shared keys for ESP/AH protection. The traditional and default transport for IKEv1 and IKEv2 is UDP port 500. During negotiation, peers exchange proposals (crypto suites), perform Diffie–Hellman to derive key material, and authenticate using pre-shared keys, certificates, or EAP methods. If a firewall blocks UDP 500, the IKE negotiation cannot begin, preventing IPsec tunnels from forming. In many real deployments, NAT traversal is also used; in that case, traffic typically shifts to UDP 4500 (NAT-T) after detection of NAT, but UDP 500 is still required for the initial exchange and NAT detection in many configurations. TCP 500 is not standard for IKE. Port 443 is associated with HTTPS/TLS and some SSL VPNs, not IPsec IKE. Therefore, among the options provided, the firewall must allow UDP 500 for IPsec key exchange to succeed.