Determine which of the given isomers of butane-2,3-diol is optically inactive. Justify your answer.

• A. Isomer (a) is optically inactive due to the presence of a plane of symmetry.
• B. Isomer (b) is optically inactive due to the presence of a plane of symmetry.
• C. Isomer (b) is optically inactive due to the absence of a chiral center.
• D. Isomer (a) is optically inactive due to the absence of a chiral center.

Answer: (B)