Preparation of p-chlorobenzoic acid from p-chloronitrobenzene
This is simply the summery of all comments involved:

$$\ce{Cl-C6H4-NO2 -&gt;[Sn/HCl] Cl-C6H4-NH3+Cl-} \tag1$$

$$\ce{Cl-C6H4-NH3+Cl- -&gt;[\text{a. } NaNO2/HCl \ (\pu{0-5 ^\circ C})][\text{b. } CuCN] Cl-C6H4-CN} \\ \tag2$$

$$\ce{Cl-C6H4-CN -&gt;[H+/H2O][\Delta] Cl-C6H4-COOH} \\ \tag3$$

Note: The reaction $(1)$ is a typical reduction reaction, which reduce nitro group to amino group. The reaction $(2)$ is called Sandmeyer Reaction. Finally, The reaction $(3)$ is a hydrolysis condition to convert cyano group to carboxylic acid group. That hydrolysis can be done under basic conditions as well (then, final product is carboxalate).



Also note that even though it is not necessary, I put this answer up just to avoid people asking same kind of questions again and again. When they have a visual answer they might not do so.