Identify the major product 'B' in the given reaction sequence.

The given reaction sequence involves two steps:

First, $p$-toluidine reacts with $\text{NaNO}_2$ and $\text{HCl}$ at $273-278\text{ K}$ to undergo diazotization. The primary amine group ($-\text{NH}_2$) is converted into a diazonium salt ($-\text{N}_2^+\text{Cl}^-$), forming $p$-toluenediazonium chloride as product A.

Next, product A is treated with hypophosphorous acid ($\text{H}_3\text{PO}_2$) and water. $\text{H}_3\text{PO}_2$ acts as a mild reducing agent, replacing the diazonium group with a hydrogen atom. The reaction proceeds as follows:
$\text{Ar}-\text{N}_2^+\text{Cl}^- + \text{H}_3\text{PO}_2 + \text{H}_2\text{O} \rightarrow \text{Ar}-\text{H} + \text{N}_2 + \text{H}_3\text{PO}_3 + \text{HCl}$

Replacing the $-\text{N}_2^+\text{Cl}^-$ group in $p$-toluenediazonium chloride with $-\text{H}$ yields toluene as the major product B.

Answer: Toluene