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

Racemisation occurs in both $\displaystyle \mathrm{S}_{\mathrm{N}} 1$ and $\displaystyle \mathrm{S}_{\mathrm{N}} 2$ reactions.

Retention occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 1$ reaction and inversion occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 2$ reaction.

Racemisation occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 1$ reaction and retention occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 2$ reaction.

Racemisation occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 1$ reaction and inversion occurs in $\displaystyle \mathrm{S}_{\mathrm{N}} 2$ reaction.