Which of the following acids does not exhibit optical isomerism?

Only those compounds exhibit optical isomerism, which have chiral centre and/or absence of symmetry elements. (Chiral carbon is the carbon all the four valencies of which are satisfied by four different groups.) (a)
(b)
(c)
(d)
Thus, maleic acid does not exhibit optical isomerism. Note If $R=H$, the $\alpha$-amino acid is achiral.