Final Answer:
Label both cis- and trans-isomers - Represents the distinct spatial arrangements of methyl groups around the carbon-carbon double bond in 2-butene for clarity and differentiation (thus the correct option is d).
Step-by-step explanation:
2-butene is an alkene molecule with four carbon atoms arranged in a chain. The cis- and trans-isomers of 2-butene differ in their geometric arrangement around the carbon-carbon double bond. In the cis-isomer, the two methyl groups are on the same side of the double bond, while in the trans-isomer, they are on opposite sides.
Drawing the structural formula for 2-butene, we represent the cis-isomer by placing the two methyl (CH3) groups on the same side of the double bond, denoted by a wedge-and-dash line drawing. The trans-isomer is depicted by positioning the methyl groups on opposite sides of the double bond, again using a wedge-and-dash line drawing to illustrate their spatial arrangement.
Therefore, to accurately represent both the cis- and trans-isomers of 2-butene, the appropriate choice is option D, which instructs to label both isomers. This way, the structural formulas will clearly demonstrate the distinct geometric arrangements of the methyl groups around the carbon-carbon double bond in each isomer (option d).