Which of the following alkanes give only ONE monochloro product (constitutional isomer) on free radical chlorination?

Count the types of H. Cyclopentane: all ten H are equivalent. 2,2-Dimethylpropane: all twelve H are equivalent. 2,2,3,3-Tetramethylbutane: all eighteen H are equivalent. 2,3-Dimethylbutane has two types (the methyl H and the tertiary H), so it gives two products. Answer: 1, 2 and 4.