Which of the following choices represents a pair of resonance forms?

• A. Cyclohexane chair and boat conformations
• B. cis-$C_2$-butene and trans-$C_2$-butene
• C. $\mathrm{Ethene}$ and $\mathrm{ethyne}$
• D. Two structures of $O_3$ where the double bond is between different pairs of oxygen atoms

Answer: (D)

Verified step by step guidance

1. Understand that resonance forms are different Lewis structures of the same molecule that differ only in the placement of electrons, not in the arrangement of atoms.
2. Evaluate the given options by checking if the structures differ only by electron placement or if they represent different molecules or conformations.
3. Recognize that cyclohexane chair and boat conformations are different spatial arrangements (conformations) of the same molecule, not resonance forms.
4. Note that cis- and trans-2-butene are stereoisomers, differing in the spatial arrangement of groups around a double bond, not resonance forms.
5. Identify that two structures of ozone (O\_3) with the double bond between different pairs of oxygen atoms are resonance forms because they differ only in electron placement while the atomic connectivity remains the same.