An unsaturated hydrocarbon is best described as which of the following?

• A. A hydrocarbon containing a benzene ring
• B. A hydrocarbon that is completely saturated with hydrogen
• C. A hydrocarbon with only single bonds between carbon atoms
• D. A hydrocarbon in which one or more bonds between carbon atoms is double or triple

Answer: (D)

Unsaturated hydrocarbons have fewer hydrogens than the maximum possible because they contain carbon–carbon multiple bonds. The key feature is one or more double or triple bonds between carbon atoms, which reduces how many hydrogens can be attached. That’s why the best description is a hydrocarbon in which one or more bonds between carbon atoms is double or triple.

The other ideas don’t fit as well: a benzene ring points to an aromatic structure, which is a specific case, not the general definition of unsaturation; a hydrocarbon that is completely saturated with hydrogen describes a saturated compound, opposite to unsaturated; and a hydrocarbon with only single bonds between carbons describes a saturated hydrocarbon.