I'd personally go for answer 'b' as it the more correct answer.
An upsloping runway will look short from an average 3 degree glideslope approach and will also make you feel like you are higher than you should be. The subsequent risk of this is that you
could make the approach too low (to give you the usual visual picture of an ideal approach) thus running the risk of landing short (which is also tested I believe). But you knew this anyway.
I don't know why your GS instructor told you anything different. Did he/she explain why you should go for answer 'a'?
If anyone else has any other view (scuse the pun!) on this question I'd be happy to stand corrected. Hopefully I passed HP&L anyway so can do a little jig when the results arrive.
VFE.