Which US department developed SAGE?

The correct answer is the Department of Defense, as it was responsible for developing the Semi-Automatic Ground Environment (SAGE) system during the Cold War. SAGE was a groundbreaking air defense system designed to provide an integrated method for detecting incoming enemy aircraft and coordinating a rapid response.

The system utilized early computer technology and advanced radar to create a network of air defense that included both ground-based control and airborne interceptors. This project was crucial for national security during a time when the threat of aerial attacks was significant.

The other departments listed did not play a role in the development of SAGE. The Department of Energy focuses on energy policies and atomic energy, the Department of Transportation deals with federal transportation policies, and the Department of Commerce manages economic growth and statistics; none of these were directly involved in military air defense technology at that time.