Good analysis there FFF.
I now think that the grey area here must come from what perspective the examiner who created this question was taking. Unfortunately perspective varies from person to person and as we all know can be affected by factors such as rain on the DV window, fog and having someone poke you in the eye etc etc.....
I would seriously go for answer 'b' still but write a little note on the plain paper to Bloggs at Aviation House explaining the reasons (as explained by FFF) for your answer...... unless LOW and LONGER appear in answer 'c' or 'd' of course.
VFE.
[Looks like FFF and I are posting at the same time here!]