What condition must be met for E/Z isomerism to occur?

E/Z isomerism occurs when there is restricted rotation around a double bond, typically a carbon-carbon double bond. For E/Z isomerism to manifest, each carbon atom involved in the double bond must be bonded to two different atoms or groups, creating distinct spatial arrangements.

In the context of option C, with each carbon bearing different substituents, this leads to the possible formations of E (entgegen) and Z (zusammen) isomers, which describe the relative positioning of the highest-priority groups attached to the double-bonded carbons. This distinction allows for the existence of geometric isomers that differ in their physical and chemical properties, such as boiling points and reactivity.

While the presence of a double bond is essential for E/Z isomerism, having either identical atoms/groups attached or only single bonds do not allow this phenomenon to showcase itself. Thus, the requirement for each carbon to be bonded to different atoms or groups effectively sets the stage for the interesting characteristics associated with E/Z isomerism.