${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Cl}\underset{\mathrm{KOH}}{\stackrel{\text{ alc }}{⟶}}\mathrm{B}\stackrel{\mathrm{HBr}}{\to }\mathrm{C}\underset{\mathrm{ether}}{\overset{\mathrm{Na}}{\to }}\mathrm{D}$

In the above sequence the product D is

The sequence of reaction can be completed as follows:-

$$\begin{gathered} {\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Cl}\underset{\mathrm{KOH}}{\stackrel{\text{ alc }}{⟶}}{\mathrm{CH}}_3-\underset{\mathrm{B}}{\mathrm{C}}\mathrm{H}={\mathrm{CH}}_2 \\ {\mathrm{CH}}_3-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2\stackrel{\mathrm{HBr}}{\to }{\mathrm{CH}}_3-\underset{\mathrm{C}}{\mathrm{C}}\mathrm{H}\left(\mathrm{Br}\right)-{\mathrm{CH}}_3 \\ {\mathrm{CH}}_3-\mathrm{CH}\left(\mathrm{Br}\right)-\mathrm{C}{\mathrm{H}}_3\underset{\mathrm{ether}}{\overset{\mathrm{Na}}{\to }}{\mathrm{CH}}_3-\underset{\mathrm{D}}{\mathrm{C}}{\mathrm{H}}_2\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_2\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_3 \end{gathered}$$

${\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Cl}$ reacts with alcoholic $\mathrm{KOH}$ and undergoes elimination reaction to give ${\mathrm{CH}}_3-\mathrm{CH}={\mathrm{CH}}_2$ which further undergoes addition reaction in presence of $\mathrm{HBr}$ to give ${\mathrm{CH}}_3-\mathrm{CH}\left(\mathrm{Br}\right)-{\mathrm{CH}}_3$. On further reaction with $\mathrm{Na}/\mathrm{ether}$ it gives the product ${\mathrm{CH}}_3-{\mathrm{CH}}_2\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_2\left({\mathrm{CH}}_3\right)-{\mathrm{CH}}_3$ i.e. 2,3-dimethyl butane.