A government worker secretly copies classified files that contain defense tactics information to an external...

The act described is espionage, where classified information is stolen and provided to adversaries or unauthorized parties, usually for political, military, or strategic advantage.

Data exfiltration (B) is the technical act of stealing data but doesn’t specify motivation. Financial gain (C) or blackmail (D) could be motivations but are not clearly indicated here.

Espionage is a classic threat actor motivation outlined in the Threats domain【6:Chapter 2†CompTIA Security+ Study Guide】.