(a)
Phenyl magnesium bromide reacts with t-butanol to produce benzene because phenyl group (electronegative group) is associated with active hydrogen of alcohol, i.e., $- H$ of $–OH$ group of alcohol.
$C_{6}H_{5}MgBr + \left( CH_{3} \right)_{3}C - OH \rightarrow C_{6}H_{5} + \left( CH_{3} \right)_{3}\ CO - MgBr$