Step 1: Reaction of p-nitrotoluene with $Cl_2/h\nu$ (side-chain chlorination) results in the substitution of a hydrogen on the methyl group. This forms p-nitrobenzyl chloride (A).
Step 2: Reaction of A with $NaI$ in dry acetone is the Finkelstein reaction.
Step 3: The chloride is substituted by iodide to form p-nitrobenzyl iodide (B).