The resistance of a wire is R ohm. If it is melted and stretched to...

Volume of wire will remain constant

So, ${A}_{1}{l}_{1} = {A}_{2}{l}_{2}$

$⇒ {A}_{2} = \dfrac{{A}_{1}}{n}$

The resistance of a wire with a length $l$, an area of $A$, and resistivity $ρ$ is stated as $R = \dfrac{ρl}{A}$.

Now, the new length is $nl$.
$∴$New resistance ${R}^{'} = \dfrac{ρnl}{A / n} ⇒ {R}^{'} = ρ{n}^{2}\dfrac{l}{A}$

$⇒ {R}^{'} = {n}^{2}R$