Final answer:
D. cyclohexane. The correct name for a six-carbon ring molecule with no additional substituents is cyclohexane, which corresponds to answer option D.
Step-by-step explanation:
The correct name of an organic molecule that has six carbon atoms in a ring is cyclohexane. The molecule described may not be explicitly stated in the question, but no additional groups like methyl or ethyl are mentioned.
Therefore, the correct answer to the question is D. cyclohexane. Organic chemistry nomenclature is very specific, and the prefix cyclo- is used when carbons are arranged in a ring structure, followed by the name of the alkane that corresponds to the number of carbon atoms.