Maleic acid and fumaric acid are:

Explanation

Option A is correct as when the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas, when the substituents are oriented in opposing directions, the diastereomer is referred to as trans. Maleic and fumaric acids are given below. Option B is incorrect as chain isomers are compounds which have the same molecular formula but the carbon skeleton of the chain is different. Example: n-pentane and 2-methyl butane Option C is incorrect as positional isomers are constitutional isomers with the same carbon skeleton and will have the same functional group. Example: 2-iodopentane and 3-iodopentane. Option D is incorrect as metamers are the isomers having the same molecular formula but different alkyl groups on two sides of functional groups. Example: Pentan -2- one and Pentan-3-one.