Explanation

(b)
The structures of maleic and fumaric acids are given below
$\ \ H - C - COOH$ $H - C - COOH$
$\ \ \ \ \ \ \ \ \ \ \ \parallel \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \parallel \$
$\ \ H - C - COOH\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ HOOC - C - H$
(maleic acid) (fumaric acid)
molecular formula ($C_{4}H_{4}O_{4})\ \ \ \$molecular formula$(C_{4}H_{4}O_{4})$
The structures of fumaric and maleic acid suggest that they are geometrical isomers because they have same molecular formula but different spatial arrangement of atoms around a double bond.