c
Resistance of a wire, \(R=\rho \frac{l}{A}=4\,\Omega\)        .... \((i)\)

When wire is stretched twice, its new length be \(l\). Then

\(l^{\prime}=2 l\)

On stretching volume of the wire remains constant.

\(\therefore l A=l^{\prime} A^{\prime}\) where \(A^{\prime}\) is the new cross-sectional area

or \(\quad A^{\prime}=\frac{l}{l^{\prime}} A=\frac{l}{2 l} A=\frac{A}{2}\)

\(\therefore \quad\) Resistance of the stretched wire is

\(R^{\prime} =\rho \frac{l^{\prime}}{A^{\prime}} =\rho \frac{2 l}{(A / 2)}=4\, \rho \frac{l}{A}\)

\(=4(4 \,\Omega) =16\, \Omega (\text { Using }(\mathrm{i}))\)