Maleic acid and fumaric acids are

 I

Because they have the same molecular formula but a different spatial arrangement of atoms around a double bond, fumaric and maleic acid appear to be geometrical isomers.

When similar groups are present on the same side of a double bond, the isomer is known as cis; when different groups are present on the same side of a double bond, the isomer is known as trans. As a result, maleic acid has a cis form and fumaric acid has a transform.