b
The resistance of a wire of length \(l\) and area

\(A\) and resistivity \(\rho\) is given as

\(R=\frac{\rho l}{A}\)

Given, \(l'=n l\)

As the volume of the wire remains constant 

\(\therefore A^{\prime} l^{\prime}=A l\)

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

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

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