The halide which will undergo $SN^2$ reaction easily

Ethyl Chloride.

The correct answer is Option (2) → Ethyl Chloride.

$SN^2$ reactions occur through a single step involving backside attack. For this, the carbon attached to the halogen must be easily accessible. Less steric hindrance increases the rate of $SN^2$ reaction.

Ethyl chloride is a primary alkyl halide. The carbon attached to chlorine is not crowded, so the nucleophile can attack easily. Hence, $SN^2$ reaction occurs readily.

Why the other options are not preferred

Tertiary butyl chloride

The carbon attached to chlorine is tertiary and highly crowded. Backside attack is not possible, so $SN^2$ does not occur.

Benzyl chloride

Although benzyl chloride can undergo $SN^2$ reaction, it also strongly favours $SN^1$ reaction due to resonance stabilisation of the benzyl carbocation. Because of this competing pathway, it is not considered the best answer when the question asks for $SN^2$ reaction easily.

Allyl chloride

Allyl chloride behaves similarly to benzyl chloride. It can undergo both $SN^1$ and $SN^2$ reactions, so it is not the most straightforward case of $SN^2$.

Final conclusion

Ethyl chloride undergoes $SN^2$ reaction most easily among the given options because it is a simple primary alkyl halide with minimal steric hindrance.