Final answer:
The change in internal energy of the gas is 0 J.
Step-by-step explanation:
The change in internal energy (ΔU) of an ideal gas during an isothermal and reversible process can be calculated using the equation: ΔU = 0
Since the gas is expanding isothermally and is ideal, there is no change in internal energy. Therefore, the answer is (a) 0 J.