Count only constitutional isomers (ignore stereoisomers). The number of different monochloro products obtainable from 2-methylbutane by free radical chlorination is

2-Methylbutane $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{CH}{}_2-\mathrm{CH}{}_3$ has four kinds of hydrogen: the two equivalent methyls on C2, the C2-H, the C3 methylene and the C4 methyl. Replacing one H of each kind gives 1-chloro-2-methylbutane, 2-chloro-2-methylbutane, 2-chloro-3-methylbutane and 1-chloro-3-methylbutane. That is 4 products.