Which description fits an alcohol?

An alcohol is specifically characterized by the presence of an –OH functional group, also known as a hydroxyl group. This group is what defines alcohols and gives them their unique properties compared to other organic compounds. For a compound to be classified as an alcohol, the –OH group must be the only functional group present, which leads to the nomenclature and behavior typical of alcohols, such as their ability to form hydrogen bonds and their solubility in water.

The other descriptions do not fit with the definition of an alcohol. A carbonyl group, defined by the C=O bond, is characteristic of aldehydes and ketones, not alcohols. Saturated hydrocarbons with a halogen would fall under the classification of haloalkanes, while the requirement for two carbon atoms is not necessary for a compound to be an alcohol; for instance, methanol (one carbon) is a simple alcohol. Thus, the specific and exclusive presence of the –OH functional group in description B identifies the compound as an alcohol.