Optically active alkyl halide undergoing S_N² substitution involves

The correct option is inversion of configuration.

Let us understand why this is the correct choice by looking at the mechanism of the S_N2 reaction:
S_N2 stands for Substitution Nucleophilic Bimolecular. It is a single-step (concerted) reaction where the bond-making and bond-breaking processes happen simultaneously.

In this mechanism, the nucleophile attacks the electrophilic carbon atom from the back side, which is directly opposite to the leaving group (the halide ion). This back-side attack minimizes steric hindrance and electrostatic repulsion between the incoming electron-rich nucleophile and the outgoing electron-rich leaving group.

As the nucleophile starts bonding to the carbon, the bond between the carbon and the leaving group begins to weaken, forming a pentavalent transition state. Once the leaving group departs, the tetrahedral geometry around the carbon atom is inverted, similar to an umbrella being blown inside out in a strong wind. This process is known as Walden inversion or inversion of configuration.

Therefore, when an optically active alkyl halide undergoes an S_N2 substitution reaction, it always leads to a complete inversion of configuration at the stereocenter.