Product ' A ' of following sequence of reactions is

In the first step ethyl benzene undergoes electrophilic substitution reaction to form 1-Bromo-4-ethylbenzene.

In the next step 1-Bromo-4-ethylbenzene undergoes free radical substitution at benzylic position followed by dehydrohalogenation(last step) in presence of alcoholic KOH to form 4-bromo styrene.