A quaternary carbon is a carbon bonded to four other carbon atoms. Which of the following alkanes contains one?

Write each structure and count the carbon neighbours of every carbon. In 2,2-dimethylbutane, C2 carries two methyl groups and is bonded to C1 and C3, so it has four carbon neighbours. In 2,3-dimethylbutane and 2-methylbutane the most-substituted carbon has three carbon neighbours, and in 3-methylpentane it also has three. Only 2,2-dimethylbutane has a quaternary carbon.