Final answer:
A. As soon as you feel resistance from the trailer. When performing the tug test, you should stop as soon as you feel resistance from the trailer. Options B, C, and D are incorrect indicators of when to stop.
Step-by-step explanation:
When performing the tug test, you should stop as soon as you feel resistance from the trailer. The tug test is a common method used to check if the trailer's brakes are working properly. By applying gradual force with your tow vehicle, you can determine if the trailer is able to hold its position and if the brakes engage when needed.
Options B and C are incorrect because the sound or pop of the tires or brakes is not a reliable indication of the trailer's braking ability. Option D is also incorrect because relying solely on a hand signal from a spotter may not provide accurate information about the trailer's braking effectiveness. Therefore, the correct answer is A, as indicated above.