The conjugate base of compound A becomes aromatic post deprotonation:
This is essentially a cyclopentadienyl anion derivative, that has 6 $\pi$ electrons that are delocalised over the ring. Thus this compound will be by far the most acidic.
As for the other cyclopentadiene derivative, it cannot be deprotonated to make it aromatic: it is localised on an $sp^2$ hybridised carbon atom, and the electron pair will thus be in the plane of the ring itself.
A similar logic can be extended to the methylated tropylium derivative. As for the tropylium derivative that can be deprotonated to give a delocalised electron system, it will have $8\pi$ electrons and thus will be antiaromatic. The molecule will thus prefer to be non-planar instead of being destabilised by the antiaromatic effect.