The correct answer is D) Place the thermostatic element in a container of water at various operating temperatures and note the movement of the valve stem. This is the correct answer because the proper way to verify the operation of a thermostatic valve is to test the thermostatic element directly by exposing it to different temperatures and observing how the valve stem moves in response. This allows you to directly evaluate the functionality of the key component that controls the valve's operation. The other options are incorrect because A) checking spring compression values would not test the thermostatic element itself, B) analyzing the power pellet chemically would not directly evaluate valve operation, and C) removing and examining the power pellet would also not test the thermostatic element's function.