All the answers look ok to me except when you ozonolyse cyclohexene you surely get a symmetrical di-aldehyde not a diketone
True, but who says A is cyclohexene? I can think of a couple of examples that follow everything else that does not start with this (one is less likely, mind)...
Cyclohexene seems the most likely to me, it can't be that complicated!
Well, as Sherlock Holmes once said "Eliminate all other factors, and the one which remains must be the truth."
If you end up with a ketone, what does that tell you about the double bond?