An alkane of molecular formula $\mathrm{C}{}_6\mathrm{H}{}_{14}$ gives exactly two different monochloro products (structural isomers only) on free-radical monochlorination. What is its IUPAC name?

Count the distinct hydrogen environments of each C6H14 isomer. Hexane has 3, 2-methylpentane has 5, 3-methylpentane has 4 and 2,2-dimethylbutane has 3. 2,3-Dimethylbutane, (CH3)2CH-CH(CH3)2, has only two: the twelve equivalent primary H and the two equivalent tertiary H. So it gives 2 monochloro products.