The netstat -a command is the correct answer because it displays all active connections and listening ports. This allows the administrator to review every connection and service that the server is involved with, which aids in identifying any unauthorized connections or unwanted services in line with the security policy.

netstat -e is incorrect as it displays Ethernet statistics, such as bytes and packets sent and received, which does not include connection or port status.

netstat -r is incorrect because it shows the routing table, which is invaluable for understanding how packets are routed through the network but does not provide detailed information regarding individual connections and listening services.

Lastly, netstat -p is incorrect in this context because it shows connections associated with the process using them, which is useful for linking network activity to specific applications or processes. However, it does not list all listening ports if not associated with an established connection.