(B) on treatment with \(\mathrm{Zn} / \mathrm{HCl}\) gives (a) aniline (b) benzylamine (c) phenylisocyanide (d) N-methylaniline

Zn/HCl is a reagent used for the reduction of various functional groups, including nitro groups. We don't have the exact structure of compound (B), but we can understand how different compounds will react with Zn/HCl based on our knowledge of organic chemistry. In this case, we will focus on the reduction of nitro groups to amines as it is a common process in organic synthesis and could be applicable to the given options. We will check each option provided in the question to see if a nitro group reduction by Zn/HCl is feasible to form the listed compounds.

(a) Aniline Aniline is formed by the reduction of nitrobenzene. Therefore, if compound (B) is nitrobenzene, the treatment with Zn/HCl would give aniline. (b) Benzylamine Benzylamine would form by the reduction of a nitrobenzyl compound, specifically, \(\mathrm{p}\)-nitrobenzyl alcohol. However, the Zn/HCl reduction requires a relatively high activation energy, and it is typically not observed, especially for \(\mathrm{p}\)-nitrobenzyl alcohol. (c) Phenylisocyanide Phenylisocyanide is formed from the hydrolysis of an isonitrile group. Zn/HCl would not reduce an isonitrile group, so this option is not feasible. (d) N-methylaniline N-methylaniline is formed by the reduction of N-methylnitrobenzene. Therefore, if compound (B) is N-methylnitrobenzene, the treatment with Zn/HCl would give N-methylaniline.

After analyzing all options, we can conclude that: - Zn/HCl can reduce nitrobenzene to form aniline (a). - Zn/HCl cannot reduce \(\mathrm{p}\)-nitrobenzyl alcohol to form benzylamine (b). - Zn/HCl cannot reduce an isonitrile group to form phenylisocyanide (c). - Zn/HCl can reduce N-methylnitrobenzene to form N-methylaniline (d). Considering the known and more likely reactions with Zn/HCl, option (a) aniline is the most probable product after treating compound (B) with the reagent.