Identify ' X ' in the following reaction.

Reaction: p-Bromo-chloro-benzene --Mg, dry ether--> intermediate --D₂O--> X

Step 1: The aryl chloride forms the Grignard reagent

p-Br–C₆H₄–Cl + Mg → p-Br–C₆H₄–MgCl

(Aryl Cl is reactive; the ring Br stays unchanged.) Step 2: Quench with heavy water (D₂O) → deuteration at the Grignard carbon

p-Br–C₆H₄–MgCl + D₂O → p-Br–C₆H₄–D + Mg(OD)Cl

• Grignard reagents act as carbanions and pick up D from D₂O.
• Result: Deuterium replaces the –MgCl site; bromine remains para.

Answer:p-bromodeutrobenzene (ring bearing Br and D para) — Option (b).