We've appealed this question several times. We thought the wording was this:
Q. Why are 'step climbs' used on long distance flights ?
(A) Step climbs do not have any special purpose for jet aeroplanes; they are used for piston engine aeroplanes only.
(B) To respect ATC flight level constraints.
(C) To fly as close as possible to the optimum altitude as aeroplane mass reduces.
(D) Step climbs are only justified if at the higher altitude less headwind or more tailwind can be expected.
Which arguably produces two possible answers. It could be (B) if the question means 'Why are 'step climbs' rather than cruise climbs used on long distance flights' and it could be (C) if they mean 'Why are 'step climbs' used on long distance flights rather than just maintaining one flight level throughout'
On balance I would have gone for (C) but there's no 'range' option in our version. Have you got a newer version of the question, Pugzi?