Option C is correct because network ACLs are evaluated in order of rule number; rule 100 allows SSH, but rule 200 denies SSH (same port, but deny overrides allow because it is evaluated later? Actually, in NACLs, rules are evaluated from lowest to highest, and the first matching rule determines the action. Since rule 100 allows, rule 200 is never reached. However, rule 300 denies all traffic.
Since rule 300 has a higher number, it will not override earlier allow rules. Wait: The correct behavior: NACL rules are processed in order; the first rule that matches the traffic determines the action. Since rule 100 matches SSH traffic and allows it, rule 200 and 300 are not evaluated for that traffic.
So SSH should be allowed. But the question says can't SSH. Let me re-check: The exhibit shows rule 100 allows SSH, rule 200 denies SSH (same criterion), rule 300 denies all traffic.
Since rule 100 matches first, SSH should be allowed. So maybe the issue is that the subnet is not associated with this NACL? Or maybe the security group is blocking? But the security group allows SSH. The most likely cause is that the NACL's inbound rule 100 allows SSH, but the outbound rules might be blocking.
However, the exhibit only shows inbound entries. Option A is incorrect because rule 300 is a deny-all, but it is evaluated after rule 100. Option B is incorrect because rule 200 is a deny, but it is not evaluated.
Option D is incorrect because security groups are stateful. Actually, the correct answer might be that the NACL's outbound rules are causing the issue, but the exhibit does not show outbound rules. However, since the question asks about the exhibit, we need to pick the best answer.
The exhibit shows only inbound rules; maybe the outbound default deny is blocking return traffic? But NACLs are stateless, so return traffic must be allowed by outbound rules. The exhibit does not show outbound rules. The default outbound rule is deny all.
So that could be the issue. But among the options, none mention outbound. Let me re-read the options.
Option A: 'The network ACL has a rule that denies all traffic (rule 300) which overrides the allow rule.' This is incorrect because rule 100 is processed first. Option B: 'The network ACL rule 200 denies SSH traffic, overriding rule 100.' Incorrect because rule 100 is processed first. Option C: 'The network ACL is not associated with the subnet.' This could be the reason.
Option D: 'The security group is blocking SSH traffic.' But the scenario says security group allows SSH. So most likely, the NACL is not associated. Therefore, option C is correct.