The correct answer is:
The volume of the gas decreases
Explanation:
As the piston is forced down work is done on the gas. The force exercised by the gas pressure on the piston is in the adverse direction of the motion of the piston. The volume of the space the gas absorbs decreases. When we let go of the piston the gas extends. The force exercised by the gas is in the similar direction as the motion of the piston and work is done by the system.