Final answer:
A universal propeller protractor should be zeroed with the aileron in the NEUTRAL position to establish a baseline for measuring its movement accurately.
Step-by-step explanation:
The subject of this question is related to aircraft maintenance and the correct use of a universal propeller protractor which is a tool used for measuring the angular movement of aircraft components such as ailerons. To ensure accurate measurements, the protractor should be zeroed correctly. The correct answer is C. with the aileron in the NEUTRAL position. This allows for a baseline to be established from which the movement of the aileron can be measured both upwards and downwards accurately. Measurements taken with the aircraft leveled and the aileron in the neutral position ensure that the readings are consistent and reliable for maintenance and alignment purposes.
By zeroing the propeller protractor with the aileron in the neutral position, any subsequent measurement of degrees of aileron travel can accurately represent the actual movement of the ailerons.