Chlorobenzene on fusing with solid $\text{NaOH}$ followed by acidification gives

The reaction can be represented as below.

${\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{Cl}+\mathrm{NaOH} \stackrel{\mathrm{fuse}}{\to } {\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{ONa} \stackrel{\mathrm{Acidification}}{\to } {\mathrm{C}}_6{\mathrm{H}}_5-\mathrm{OH}$

In the above reaction, Phenol is formed as the final product.