meso-Tartaric acid, (2R,3S)-2,3-dihydroxybutanedioic acid, is optically inactive although it has two stereocentres. This is because

Ait is an equimolar mixture of two enantiomers
Bit has an internal plane of symmetry, so the rotation due to one half is cancelled by the other half (internal compensation)
Cit has no chiral carbon atom
Dits two carboxyl groups are identical so the molecule is planar

Answer: B

The (2R,3S) isomer has a mirror plane through the midpoint of the C2-C3 bond in the eclipsed conformation, so it is superposable on its mirror image (achiral). Its inactivity is intramolecular (internal compensation), not due to mixing of two species.

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