SN2 (Substitution Nucleophilic Bimolecular) reactions occur at an asymmetric carbon when a nucleophile attacks the carbon atom, resulting in the substitution of a leaving group. In such reactions, there are four possible outcomes:

a) An enantiomer of the substrate: In an SN2 reaction, the nucleophile attacks the carbon atom from the backside, causing inversion of configuration. As a result, the product formed is the enantiomer of the substrate. This occurs when the nucleophile is a strong base and the leaving group is a good leaving group.

b) A product with opposite optical rotation: Optical rotation is a measure of the rotation of polarized light by a chiral compound. In an SN2 reaction, when a nucleophile attacks an asymmetric carbon, the product formed may have a different optical rotation compared to the substrate. This occurs when the nucleophile introduces a new chiral center or changes the existing chiral center's configuration.

c) A mixture of diastereomers: Diastereomers are stereoisomers that are not mirror images of each other. In an SN2 reaction, if the nucleophile attacks an asymmetric carbon and forms a new chiral center, the product may exist as a mixture of diastereomers. This occurs when the nucleophile is a chiral reagent or the reaction conditions favor the formation of multiple products.

d) A single stereoisomer: The correct answer is option 'D'. In some SN2 reactions, the reaction conditions and the nature of the nucleophile and leaving group may lead to the formation of a single stereoisomer. This occurs when the reaction is highly selective, and the nucleophile attacks the carbon atom with a specific orientation, resulting in the formation of a single stereoisomer.

Overall, the outcome of an SN2 reaction at an asymmetric carbon depends on various factors such as the nature of the nucleophile, leaving group, reaction conditions, and the presence of other chiral centers. While options a, b, and c are possible outcomes, option d (a single stereoisomer) is the correct answer as it represents a specific scenario where only one stereoisomer is formed.

SN2 involves backside attack of the nucleophile & hence inversion of configuration occurs.