$A$ and resistivity $\rho$ is given as

$R=\frac{\rho l}{A}$

Given, $l'=n l$

As the volume of the wire remains constant

$\therefore A^{\prime} l^{\prime}=A l$

$A^{\prime}=\frac{A l}{l^{\prime}}=\frac{A l}{n l} \text { or } A^{\prime}=\frac{A}{n}$

$ \therefore \quad R^{\prime} =\frac{\rho l^{\prime}}{A^{\prime}} $

$ R^{\prime} =\frac{\rho n l}{\frac{A}{n}}=\frac{n^{2} \rho l}{A}=n^{2} R $