A nucleophilic substitution reaction is a chemical reaction in which an electron rich molecule (nucleophile) replaces a functional group in an electron deficient molecule (electrophile). Carbocation formation occurs in SN$^1$ mechanism (unimolecular nucleophilic substitution).

Carbocation is formed in the first step when the bond between carbon and the leaving group breaks. The nucleophile attack on the carbocation occurs in the second step.

The general stability order of carbocation is

Tertiory > Secondary > Primary

The stability order including allyl carbocation, benzyl carbocation, phenyl carbocation and triphenyl methyl $$\left( {{{(Ph)}_3}{C^ + }} \right)$$ carbocation is

Phenyl < Allyl < Benzyl < Triphenyl methyl

(1)

Carbocation formed from this halide is

Allylic carbocation stability is due to the resonance delocalization of the positive change across the adjacent double bond.

(2)

Stable due to the delocalization of positive change across all three phenyl rings.

(3)

Benzyl carbocation stability is because of the positive charge delocalization across the aromatic ring. So, it is mole stable than allylic carbocation but less stable than Ph$_3$C+ carbocation.

(4)

Delocalization is possible for the adjacent double bond. So, most stable carbocation is

The answer is option (2)