The compound shown is X, a stereoisomer of 3-chlorobutan-2-ol. Which of the structures (A) to (D) shown with the options are diastereomers of X?

X is (2S,3R)-3-chlorobutan-2-ol. (A) is (2R,3S): both centres inverted, so it is the enantiomer of X. (B) is (2S,3S): only C3 inverted, so a diastereomer. (C) is drawn from the other end of the chain but is (2S,3R), identical to X. (D) is (2R,3R): only C2 inverted, so a diastereomer. 3-Chlorobutan-2-ol has two different stereocentres, so there is no meso form and the four stereoisomers form two enantiomeric pairs.