Optical isomerism is a form of stereoisomerism exhibited by molecules that are chiral (non-superimposable on their mirror images), typically due to the presence of an asymmetric carbon atom bonded to four different groups.

1. Maleic acid (cis-but-2-enedioic acid) possesses a plane of symmetry and lacks a chiral center. It exhibits geometrical isomerism (cis-trans) along with Fumaric acid, but it is optically inactive.
2. Lactic acid (2-hydroxypropanoic acid) contains one chiral carbon atom and exists as a pair of enantiomers.
3. Tartaric acid (2,3-dihydroxybutanedioic acid) contains two chiral centers and exhibits optical isomerism (d-form, l-form, and meso-form).
4. $\alpha$-amino acids (except glycine) contain a chiral $\alpha$-carbon atom and are optically active.