Maleic acid and fumaric acids are

Maleic acid and fumaric acid are two different arrangements of the same molecular formula $$C_4H_4O_4$$ (the structure of $$HOOC - CH = CH - COOH$$).

Because the C=C double bond cannot freely rotate, the two $$-COOH$$ groups can be located either on the same side of the double bond (cis) or on opposite sides (trans).

This difference arises purely from restricted rotation about the double bond, not from any change in carbon chain, functional group, or tautomeric shift. Hence they are classified as geometrical (cis-trans) isomers.

Therefore, the correct choice is:
Option D which is: Geometrical isomers