a
The reactivity of the carbonyl group towards the addition reactions depends upon the magnitude of the positive charge on the carbonyl carbon atom. Hence aryl substituent that increases the positive charge on the carbonyl carbon must increase its reactivity towards addition reactions. The introduction of negative group (\(-I\) effect) increases the reactivity while introduction of alkyl group (\(+I\) effect) decreases the reactivity.

\(+ I\) effect and steric hindrance increases