Which compound is used to denote the presence of an alcohol functional group in organic chemistry?

The notation R-OH is the correct representation of an alcohol functional group in organic chemistry. Here, "R" represents an organic substituent, which can be a hydrocarbon chain, while "OH" indicates the presence of a hydroxyl group. The hydroxyl group is the defining feature of alcohols, and its connection to the carbon chain (represented by "R") is what categorizes the molecule as an alcohol.

In the context of the other options, R2CO describes a ketone functional group, RC(=O)R' refers to the structure of a carbonyl group primarily found in ketones and aldehydes, where "C(=O)" indicates a carbon double-bonded to oxygen. RCOOH denotes a carboxylic acid, characterized by both a carbonyl and a hydroxyl group but arranged in a distinct manner that defines the acid's properties. Therefore, B, R-OH, is uniquely suited to denote the alcohol functional group, making it the correct choice.