Step 1: Understanding the Concept:
This is a sequence of reactions: diazotization followed by a Sandmeyer reaction.
Step 2: Detailed Explanation:
1. Aniline (C\(_6\)H\(_5\)NH\(_2\)) reacts with NaNO\(_2\) and HCl at 273-278 K (0-5\(^\circ\)C) to form benzene diazonium chloride, which is compound (Y). The reaction is diazotization.
2. When benzene diazonium chloride is treated with Cu/HBr (a Sandmeyer reaction), the diazonium group (-N\(_2^+\)) is replaced by a bromine atom (-Br), forming bromobenzene (C\(_6\)H\(_5\)Br), which is compound 'Z'.
Step 3: Final Answer:
Compound 'Z' is bromobenzene.