Which switch activates the fresh air blower while on the ground?

The switch that activates the fresh air blower while on the ground is the one related to the weight of the landing gear. This switch is designed to ensure that the fresh air system operates only when the aircraft is on the ground and the landing gear is down and locked. This prevents the fresh air blower from running during flight, maintaining system efficiency and safety by ensuring that the aircraft's cabin stays ventilated only when necessary.

Operating the fresh air blower while the aircraft is on the ground helps to maintain cabin comfort for the crew and passengers before takeoff. In contrast, the other options pertain to different functions: the engine start switch initiates engine operation, the emergency power switch is related to backup systems, and the fresh air control switch typically adjusts airflow rather than activating the blower itself. Thus, the landing gear weight-on-wheels switch is specifically responsible for activating the fresh air blower in ground operations.