The most reactive compound among the following toward nucleophilic addition reaction is:

1. 2.
3. 4.

Hint: Aldehyde is more reactive than ketone

The reactivity of aldehyde is more than ketone because in ketone carbonyl carbon is attached to two bulky groups which sterically hindered the attack of nucleophile. Thus, we can ruled out option second and fourth.

In ethanal and benzaldehyde, ethanal is more reactive towards  nucleophilic addition reaction because the partially positive charge in carbonyl carbon of ethanal is more than benzaldehyde. Carbonyl carbon of benzaldehyde partially positive charge is more stable due to resonance effect of benzene ring. Hence option first is the correct answer.