Which probe or mast is NOT heated on the ground when at least one engine is running?

The total air temperature (TAT) probe is designed to be heated at all times when the aircraft is in operation and at least one engine is running. This is critical for preventing any ice accumulation that could affect its functioning and, consequently, the accuracy of the data it provides regarding air temperature, which is essential for flight safety.

In contrast, both the pitot probes and the angle of attack probes are heated while the aircraft is on the ground with engines running to prevent ice buildup, which is vital for maintaining accurate readings of airspeed and aircraft attitude during flight. The forward lavatory drain mast, typically located at a lower point on the aircraft, does not require heating in the same manner as the performance-critical probes, but it is not generally categorized as a critical instrument for flight safety related to aerodynamic measurements like the TAT, pitot, and angle of attack probes.

Thus, since the TAT probe remains unheated when the aircraft is on the ground with at least one engine running, it is the correct choice.