In the following reaction, the absolute configurations of the chiral centres in X and Y are A. 2S, 3R and 2R, 3R B. 2R, 3R and 2R, 3S C. 2S, 3S and 2R, 3R D. 2S, 3R and 2S, 3R

The correct answer is A. 2S, 3R and 2R, 3R.

The absolute configuration of a chiral center is the spatial arrangement of the four groups attached to the chiral center. The absolute configuration is designated by the R or S system, which is based on the Cahn-Ingold-Prelog priority rules.

In the reaction shown, the chiral center in X is the carbon atom with the hydroxyl group attached. The priority of the groups attached to this carbon atom is as follows:

1. Hydroxyl group (-OH)
2. Methyl group (-CH3)
3. Ethyl group (-CH2CH3)
4. Hydrogen atom (-H)

The absolute configuration of the chiral center in X is R, because the hydroxyl group is in the R configuration.

The chiral center in Y is the carbon atom with the bromide group attached. The priority of the groups attached to this carbon atom is as follows:

1. Bromide group (-Br)
2. Methyl group (-CH3)
3. Ethyl group (-CH2CH3)
4. Hydrogen atom (-H)

The absolute configuration of the chiral center in Y is R, because the bromide group is in the R configuration.

Therefore, the absolute configurations of the chiral centers in X and Y are 2S, 3R and 2R, 3R, respectively.