Answer:
B. An isothermal compression of an ideal gas.
Explanation:
The internal energy of an ideal gas is just function of the temperature; it does not matter what other thermodynamic property changes, if the temperature does not change, the internal energy neither does. That is just for ideal gases; real gases behaviour is not like that. All of the other options bring with them an increase or decrease of the temperature:
For A, the temperature will decrease because the gas will do work as it expands, converting part of his internal energy to work.
For C, the temperature will increase, because given , if the volume increases (expansion) and the pressure is constant, the temperature must increase to satisfy the equation.
