Which of the following acids does NOT exhibit optical isomerism?

Optical isomerism requires the presence of a chiral center (a carbon atom bonded to four different groups).Lactic acid (${\text{CH}}_3\text{CH(OH)COOH}$): Has a chiral carbon.Tartaric acid ($\text{HOOC-CH(OH)-CH(OH)-COOH}$): Has chiral carbons.$\alpha \text{-amino acids}$ (except glycine): Have chiral carbons.Maleic acid ($\text{HOOC-CH=CH-COOH}$): It is an unsaturated dicarboxylic acid with a double bond. It exists as a cis isomer but contains no chiral carbon atom. Therefore, it is optically inactive.