See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Tartaric acid (2,3-dihydroxybutanedioic acid) has two chiral centers (C2 and C3), each bearing OH, H, COOH, and the rest of the chain as substituents. To enumerate all stereoisomers: with two chiral centers, a maximum of 2^2 = 4 stereoisomers are conceivable. However, because both chiral centers bear identical substituents, one combination produces a meso compound (the (2R,3S) or equivalently (2S,3R) form), which has an internal plane of symmetry and is therefore optically inactive. The remaining stereoisomers are (2R,3R)-tartaric acid (D-tartaric acid) and (2S,3S)-tartaric acid (L-tartaric acid), which are non-superimposable mirror images (an enantiomeric pair) and are both optically active. Thus, out of the three distinct compounds (D-tartaric acid, L-tartaric acid, and meso-tartaric acid), exactly two are optically active. The meso form is excluded because internal symmetry cancels optical activity. Option (a) 1 is too few; option (c) 3 counts the meso form incorrectly as optically active; option (d) 4 overcounts. Therefore, the correct answer is B.