Final answer:
Cyclohexane cannot exist as cis/trans isomers as it does not contain a double bond and has identical substituents at each carbon of the ring which makes cis/trans isomerism impossible.
Step-by-step explanation:
The question asks which of the following compounds cannot exist as cis/trans isomers: A) 2-butene, B) 2-methylbutene, C) 2,3-dimethylbutene, and D) Cyclohexane. The formation of cis/trans isomers, or geometric isomers, is dependent on the presence of a C=C double bond with different substituents on each carbon atom of the double bond, which prohibits free rotation and allows for two distinct spatial arrangements of the substituents.
Cyclohexane (D), however, does not contain a double bond and is instead a cyclic compound. While cyclic compounds can have cis/trans configurations, especially if they contain double bonds, a simple ring like cyclohexane without any substituent diversity at the relevant carbon atoms or without double bonds cannot exhibit cis/trans isomerism. Hence, cyclohexane cannot exist as cis/trans isomers due to the absence of a double bond and identical substituents around any of the carbon atoms that could give rise to such isomerism.