${\mathrm{CH}}_2={\mathrm{CH}}_2+{\mathrm{Br}}_2\stackrel{{\mathrm{CCl}}_4}{⟶}\mathrm{X}\stackrel{\text{ aq KOH }}{⟶}\mathrm{Z}$

In the above reaction $Z$ is

According to given reaction sequence,

$$\begin{gathered} {\mathrm{CH}}_2={\mathrm{CH}}_2+{\mathrm{Br}}_2\to \mathrm{CH}\left(\mathrm{Br}\right)-\mathrm{CH}\left(\mathrm{Br}\right) \left(\text{ in presence of }{\mathrm{CCl}}_4\right) \\ \mathrm{CH}\left(\mathrm{Br}\right)-\mathrm{CH}\left(\mathrm{Br}\right)\stackrel{\mathrm{aqKOH}}{\to }\mathrm{CH}\left(\mathrm{OH}\right)-\mathrm{CH}\left(\mathrm{OH}\right) \end{gathered}$$

Thus, $\mathrm{Z}$ is $\mathrm{CH}\left(\mathrm{OH}\right)-\mathrm{CH}\left(\mathrm{OH}\right)$, which is known as ethylene glycol.