Final answer:
For an adiabatic process in an ideal monatomic gas, where the insulated piston moves downward, the gas's internal energy increases due to work done on the gas by the piston.
Step-by-step explanation:
When discussing the behavior of an ideal monatomic gas in an insulated container with an insulated, movable piston moving downward, some principles of thermodynamics apply. For such an ideal gas, the internal energy is a function of temperature alone. In an adiabatic process, no heat is exchanged with the surroundings. Therefore, if the piston is moving downwards, it is compressing the gas, which would typically lead to an increase in the gas's temperature and thus an increase in internal energy.
Given that the container and piston are insulated, we can assume that the process is adiabatic. During the adiabatic compression, no heat is transferred to or from the gas. Instead, the work done on the gas by the piston results in an increase in internal energy, represented by the increase in temperature (although the problem statement specifically asks to ignore changes in temperature). So the correct answer to the student's question would be C. Increase in internal energy.