An alkyl bromide $\mathrm{R}-\mathrm{Br}$ is converted by the Wurtz reaction (Na, dry ether) into an alkane $\mathrm{R}-\mathrm{R}$. If $\mathrm{R}-\mathrm{R}$ gives exactly two constitutional monochlorination products, $\mathrm{R}-\mathrm{Br}$ is:

1-Bromopropane gives hexane (3 types of H, 3 products). 2-Bromopropane gives 2,3-dimethylbutane, with only $\mathrm{CH}{}_3$ and $\mathrm{CH}$ hydrogens, so 2 products. 1-Bromo-2-methylpropane gives 2,5-dimethylhexane ($\mathrm{CH}{}_3$, $\mathrm{CH}{}_2$, $\mathrm{CH}$), so 3 products. Bromomethane gives ethane with 1 product.