Which NAVAID cannot be used as a NAV source on the T-54?

The selection of NDB as the correct answer is based on the T-54's navigation capabilities and the types of navigational aids available for utilization in that system.

NDB, or Non-Directional Beacon, is a type of radio transmitter used in aviation to provide navigational guidance. However, the T-54 does not employ NDB as a navigational source because it primarily relies on more advanced systems that provide precision and reliability in navigation, such as TAC, VOR, and LOC systems.

On the other hand, TAC (Tactical Air Navigation) is a military navigation system designed to assist with navigation in combat situations; VOR (VHF Omnidirectional Range) is a standard radio navigation system for aircraft; and LOC (Localizer) is a component of an instrument landing system that provides lateral guidance to a runway. Each of these systems provides precise and effective means for navigation, making them suitable NAV sources for the T-54.

In summary, the T-54 is not configured to utilize the NDB as a navigation source, making it the correct choice in this context.