The Attempt at a Solution
Firstly, since -Br is the better leaving group among the three, it's either (a) or (b). Further, since -Br is present at α position in (b), which also has an acidic α hydrogen to the same carbon, it would react better with a weak nucleophile (favouring SN1) than in (a). So, the answer's (b). But is my line of action correct?