A tertiary alcohol is defined by:

A tertiary alcohol is characterized by having three alkyl groups attached to the carbon atom that is bonded to the hydroxyl (–OH) group. This specific structural arrangement is what distinguishes tertiary alcohols from primary and secondary alcohols.

In primary alcohols, the carbon with the -OH group is attached to only one alkyl group, while in secondary alcohols, it is attached to two alkyl groups. Tertiary alcohols have the most branching because the central carbon is connected to three other carbon atoms, leading to unique reactivity patterns and properties compared to their primary and secondary counterparts.

Thus, when defining a tertiary alcohol, the correct understanding hinges on the identity of the carbon attached to the hydroxyl group: it must indeed be surrounded by three alkyl groups, which is precisely what the correct answer states.