What is the most suitable term for the following transformation?

The correct answer is Option (4) → Retention of configuration

In the transformation, the group X on the asymmetric carbon is replaced by Y, and Y takes the same position that X held. The groups A, B and C keep their spatial arrangement, so the product has the same configuration as the reactant. This is retention of configuration.

Why the other options are incorrect

Option (1) Inversion of configuration — Incorrect. In inversion, Y enters from the side opposite to X and the other three groups flip over, as in an S_N2 reaction; here they do not.

Option (2) Racemisation — Incorrect. Racemisation gives a 50:50 mixture of the retained and inverted products; only one product is shown here.

Option (3) Elimination reaction — Incorrect. Elimination would remove X together with a β-hydrogen to form a C=C bond; here X is replaced by Y, which is substitution.

NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “(iii) Retention: Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.”