The correct answer is to configure an 'implicit deny' rule at the end of the access control list (ACL). This rule ensures that if traffic does not match any of the previous allow rules, it will be denied by default, thus providing an additional layer of security. This is the most fitting option since it aligns with the concept of 'deny by default' where everything not explicitly allowed is denied. Other options such as 'allow by default' or 'ignore unspecified traffic' would result in a less secure configuration since they could potentially let unwanted or unverified traffic through the firewall, which is against the security best practices associated with an implicit deny approach.