Rearrangement of anions
The most fascinating thing about aldol reaction is that it is reversible. So, when the aldol product is kept in a basic medium, it can actually undergo reverse mechanistic pathway to regenerate the two carbonyl compounds between which condensation occured to form the product . Then those two carbonyl compounds can literally undergo condensation in a different way to produce some different aldol product. This phenomenon is called as Retro-aldol reaction.  

In this case, also if you observe carefully, the compound (A) is an $\alpha-\beta $ unsaturated ketone (basically a dehydrated aldol product), which by hydrolysis can convert itself into an aldol and thus undergo retro-aldol reaction. Here the aldol condensation was done intramolecularly, so the two ketones produced undergoes intramolecular aldol condensation in a different way to produce another isomer (B). 

The mechanism of formation of (B) is as follows, 


Thus, option (a) should be the correct answer.