Network ACLs are stateless, meaning both inbound and outbound rules must explicitly allow traffic. While inbound HTTP traffic (port 80) was allowed, the return traffic from the EC2 instance uses ephemeral ports (typically 1024–65535). If outbound rules do not allow this ephemeral port range, the response traffic is dropped, preventing the website from loading.

Security groups are stateful and automatically allow return traffic, but network ACLs do not. This commonly causes connectivity issues when custom ACLs are applied without matching outbound rules.

Option B is incorrect because security groups allow all outbound traffic by default. Option C is irrelevant. Option D is incorrect because only one network ACL can be associated with a subnet at a time.

Thus, the missing outbound ephemeral port rule in the network ACL is the root cause.