Final answer:
Among the provided compounds, cyclohexane is the nonaromatic one because unlike benzene, naphthalene, and pyridine, it does not contain a benzene ring or a delocalized π-electron system.
Step-by-step explanation:
To determine which one of the following compounds is nonaromatic, we must understand the criteria for a compound to be classified as aromatic. An aromatic compound typically has a cyclic structure with conjugated π-electrons that follow Hückel's rule (4n+2 π-electrons for some integer n). Among the options provided, benzene and pyridine both contain a benzene ring, which is the prototypical aromatic system, and naphthalene also has a conjugated system with multiple benzene rings, making them aromatic. In contrast, cyclohexane is a saturated hydrocarbon with only single bonds; it does not have a delocalized π electron system and does not meet the criteria for aromaticity. Therefore, cyclohexane is the nonaromatic compound among the options provided.