Product 'A' of following sequence of reactions is

This is an electrophilic aromatic substitution bromination of the benzene ring.
Due to steric hindrance at the ortho-position, the para-product is formed as the major product.

This condition favors free-radical substitution rather than aromatic substitution, targeting the alkyl side chain.

The base abstracts a proton from the adjacent $$\beta$$-carbon (the $$-\text{CH}_3$$ group), and the chlorine atom leaves, introducing a carbon-carbon double bond ($$\text{C}=\text{C}$$).
This forms a conjugated alkene system (styrene derivative) which is exceptionally stable due to resonance with the benzene ring.
Hence, Option D is correct.