An S_N2 reaction at an asymmetric carbon of a compound always gives

a single isomer

The answer is (4) a single isomer.

An S_N2 reaction at an asymmetric carbon of a compound always gives a single isomer. This is because the reaction proceeds through a concerted mechanism, which means that the nucleophile attacks the carbon atom at the same time as the leaving group leaves. This results in the inversion of the stereochemistry at the carbon atom.

The other options are incorrect because they are not always produced in an S_N2 reaction at an asymmetric carbon of a compound.

Option (1) is incorrect because an S_N2 reaction does not always produce an enantiomer of the substrate. For example, if the substrate is a racemic mixture, the product will also be a racemic mixture.

Option (2) is incorrect because an S_N2 reaction does not always produce a product with opposite optical rotation. For example, if the substrate is a meso compound, the product will be optically inactive.

Option (3) is incorrect because an S_N2 reaction does not always produce a mixture of diastereomers. For example, if the substrate is a chiral compound with no chiral centers adjacent to the asymmetric carbon, the product will be a single isomer.