Consider the given reaction sequence and identify the final major...

The given starting material is p-toluenediazonium chloride.

In step (i), the reaction with $\mathrm{Cu}_2\mathrm{Cl}_2 / \mathrm{HCl}$ is a Sandmeyer reaction. The diazonium group ($-\mathrm{N}_2^+\mathrm{Cl}^-$) is replaced by a chlorine atom to form p-chlorotoluene.

In step (ii), p-chlorotoluene reacts with magnesium ($\mathrm{Mg}$) in the presence of dry ether to form a Grignard reagent, p-tolylmagnesium chloride.

In step (iii), the Grignard reagent undergoes an acid-base reaction with heavy water ($\mathrm{D}_2\mathrm{O}$). The nucleophilic carbon attached to magnesium abstracts a deuterium ion ($\mathrm{D}^+$) from $\mathrm{D}_2\mathrm{O}$, resulting in the formation of p-deuterotoluene as the final major product X.

Answer: p-Deuterotoluene