Which classification of alcohol involves two alkyl groups attached to the carbon bonded to the hydroxyl?

The classification of alcohol that involves two alkyl groups attached to the carbon bonded to the hydroxyl group is secondary (2º) alcohol. In a secondary alcohol, the carbon atom that is directly connected to the hydroxyl group (-OH) is also bonded to two other carbon atoms, which are the alkyl groups. This structural feature distinguishes secondary alcohols from primary and tertiary alcohols.

Primary alcohols have the hydroxyl-bearing carbon attached to only one other carbon, while tertiary alcohols have the hydroxyl-bearing carbon attached to three other carbons. Quaternary carbon is not applicable in this context since it refers to a carbon atom bonded to four other carbon atoms, which cannot have a hydroxyl group due to tetravalency. Thus, the defining characteristic of secondary alcohols is clearly the presence of two alkyl groups attached to the relevant carbon.