Which one of the following acids does not exhibit optical isomerism?

To determine which acid does not exhibit optical isomerism, we must recall that optical isomerism arises when a molecule has a chiral center, typically a carbon atom with four different substituents, and the molecule lacks a plane of symmetry. A chiral molecule is non-superimposable on its mirror image. Let's examine each option step by step.

Starting with option A: Lactic acid. Its molecular formula is $$CH_{3}CH(OH)COOH$$. The central carbon atom (the one attached to the OH group) is bonded to four different groups: a hydrogen atom (H), a hydroxyl group (OH), a methyl group ($$CH_{3}$$), and a carboxyl group ($$COOH$$). Since all four groups are distinct, this carbon is a chiral center. Therefore, lactic acid exhibits optical isomerism and exists as two enantiomers.

Next, option B: Tartaric acid. Its molecular formula is $$HOOCCH(OH)CH(OH)COOH$$. This molecule has two carbon atoms, each bonded to an OH group. Each of these carbons is attached to four different groups: H, OH, COOH, and the other $$CH(OH)COOH$$ group. Thus, both carbons are chiral centers. Tartaric acid exists in three stereoisomeric forms: the d-form, the l-form (which are enantiomers and optically active), and the meso-form (which has a plane of symmetry and is optically inactive due to internal compensation). However, since tartaric acid can exist in optically active forms (the d- and l-enantiomers), it does exhibit optical isomerism.

Now, option C: Maleic acid. Maleic acid is the cis-isomer of butenedioic acid, with the formula $$HOOCCH=CHCOOH$$. The structure can be represented as:

$$\begin{array}{c} \text{H} & & \text{COOH} \\ & \backslash & \\ & C = C \\ & / & \\ \text{COOH} & & \text{H} \end{array}$$

In maleic acid, the double bond between the two carbon atoms restricts rotation. Each carbon in the double bond is sp² hybridized and bonded to three atoms: one hydrogen (H), one carboxyl group (COOH), and the other carbon via the double bond. Since neither carbon has four substituents, there are no chiral centers. Additionally, the molecule is planar and has a plane of symmetry that bisects the double bond and aligns the two identical halves, making it achiral. Therefore, maleic acid does not exhibit optical isomerism.

Finally, option D: α-amino acids. These have the general formula $$R-CH(NH_{2})COOH$$, where the α-carbon is bonded to four groups: a hydrogen atom (H), an amino group ($$NH_{2}$$), a carboxyl group ($$COOH$$), and a side chain (R). For most α-amino acids, R is different from H, $$NH_{2}$$, and $$COOH$$, making the α-carbon a chiral center. For example, alanine (R = $$CH_{3}$$) has a chiral center and exhibits optical isomerism. However, glycine is an exception where R = H, so the α-carbon has two identical H atoms and is not chiral. Despite glycine not exhibiting optical isomerism, the class of α-amino acids as a whole includes compounds that do exhibit optical isomerism (like alanine). Therefore, this option does not universally lack optical isomerism.

Comparing all options, lactic acid (A) and tartaric acid (B) exhibit optical isomerism due to chiral centers. α-amino acids (D) include some that exhibit optical isomerism. Maleic acid (C) has no chiral center and is achiral, so it does not exhibit optical isomerism.

Hence, the correct answer is Option C.