Maleic acid and fumaric acid are ......... isomers.

Explanation

To determine the type of isomers that maleic acid and fumaric acid are, we need to analyze their structures and the nature of their isomerism.

### Step-by-Step Solution:

1. **Identify the Structures**:
- Maleic acid (cis-butenedioic acid) has the two carboxylic acid groups (-COOH) on the same side of the double bond.
- Fumaric acid (trans-butenedioic acid) has the two carboxylic acid groups on opposite sides of the double bond.

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