Bromination of carbonyl compound at $\alpha$ position is

$\begin{array}{1 1}a)\;Nucleophilic\; addition\\b)\;Nucleophilic\; substitution\\c)\;Electrophillic\; addition\\d)\;Electrophilic\; substitution\end{array}$

The carbon bears a negative charge and attracts nucleophiles
Hence (b) is the correct answer.