The correct statement regarding nucleophilic substitution reaction in a chiral alkyl halide is :

In nucleophilic substitution reactions of chiral alkyl halides:

$$S_N1$$ reaction: Proceeds through a planar carbocation intermediate. The nucleophile can attack from either side, leading to racemisation.

$$S_N2$$ reaction: Proceeds through a backside attack mechanism (concerted). The nucleophile attacks from the opposite side of the leaving group, leading to inversion of configuration (Walden inversion).

The answer is: Racemisation occurs in $$S_N1$$ reaction and inversion occurs in $$S_N2$$ reaction, which corresponds to Option (4).