Optically active alkyl halide undergoing $S_N2$ substitution involves

inversion of configuration

The correct answer is Option (3) → inversion of configuration

• SN2 reaction is a bimolecular nucleophilic substitution.
• The nucleophile attacks from the backside of the carbon attached to the leaving group.
• This leads to Walden inversion, i.e., the configuration of the chiral center inverts.
• No carbocation intermediate is formed, so racemization does not occur.

✔ Hence, the correct option is inversion of configuration.