Most acidic compound amongs the following unsaturated ketones?
Its not because of any ring expansion but because of formation of an aromatic compound. 

Lets Start with (C) and (D). Both compounds after formation of carbanion are stabilized by extended conjugation:



But (A) forms an even more stable carbanion because of aromatic nature of:



Cyclopentadiene has a $p_{ka}$ of 16. Its high acidic strength is because of the above reason. 

You can analyse (B) yourself. But it wont form a carbanion as stable as (A).