Which of the following should an organization use to protect its environment from external attacks conducted by an unauthorized hacker?

A Network Intrusion Prevention System (NIPS) is the most effective choice for actively protecting an environment from external attacks by an unauthorized hacker. A NIPS monitors network traffic for malicious activity and can automatically block or prevent detected threats in real-time. An Access Control List (ACL) defines permissions but does not actively detect or prevent attacks. An Intrusion Detection System (IDS) monitors and alerts on suspicious activity but does not automatically block threats; it's a passive system. A Host-based Intrusion Detection System (HIDS) monitors individual hosts, not the network perimeter, and also only detects, rather than prevents, attacks.