Which statement describes cis-trans isomerism?

• A. A type of stereoisomerism arising from restricted rotation around a bond
• B. A form of structural isomerism
• C. A property only seen in cyclohexane
• D. A phenomenon where isomers have different molecular formulas

Answer: (A)

Cis-trans isomerism is a type of stereoisomerism that arises because rotation around a bond is restricted, so the spatial arrangement of substituents can be fixed rather than freely interchanging.

This restriction happens most clearly with a carbon–carbon double bond, where rotation is not possible, and in cyclic compounds where the ring itself blocks rotation. If each carbon of the double bond (or the ring) bears different substituents, two distinct arrangements exist: substituents on the same side (cis) or on opposite sides (trans). Because the two forms have the same connectivity and the same molecular formula, they are stereoisomers rather than structural isomers.

So the best description is that cis-trans isomerism is a form of stereoisomerism arising from restricted rotation around a bond. It’s not about structural connectivity changes, it’s not limited to cyclohexane, and it doesn’t involve different formulas.