First, differentiate between efficiency and thrust. You can have a grossly inefficient engine producing lots of thrust. Second, efficiency is always stated as a comparison - SFC is a good example, fuel flow against thrust produced.
So the question starts of badly. What is "the efficency" of a jet engine? We could look at thermal efficiency, volumetric efficiency or propulsive efficiency, to name but a few. But in this context - ATPL questions - it probably means SFC
So how do the answers stack up? Decreased intake temp does two things. It increases density and therefore mass flow and thrust but also increases thermal efficiency. Themal efficiency is driven by the difference between the fixed EGT and the inlet temp. Volumetric efficiency is driven by engine design and is usually a maximum at about 90%N1. It is, therefore under your control to some degree but I don't think the examiner means that. At the back of the examiner's mind he is linking this with the conditions for max range, when you would use 90%N1 as a given. That leaves increased pressure, which would increse density and therefore mass flow and thrust but will have no direct efect on "efficiency"
A bit woolly, I'm afraid, but I hope it helps.