There were many comments on this question from the teachers, ranging from it being unfair to it being impossible. The physics of the situation is clear, though. The process is adiabatic, as there is no thermal energy transfer involved. It is also reversible. Hence there is no change of entropy either in the gas or the surroundings (Entropy change = ΔQ/T). Alternatively it can be argued that the total entropy cannot decrease, hence, by elimination, D must be the best answer. This question is covered by the syllabus item 10.3.3 and was deemed fair.