b
$V=I R=\left(n e A v_{d}\right) \rho \frac{\ell}{A}$
$\therefore \quad \rho=\frac{\mathrm{V}}{\mathrm{V}_{\mathrm{d}} \mathrm{lne}}$
Here $\mathrm{V}=$ potential difference
$\mathrm{I}=$ length of wire
$\mathrm{n}=\mathrm{no} .$ of electrons per unit volume of conductor.
$\mathrm{e}=\mathrm{no.}$ of electrons
Placing the value of above parameters we get resistivity
$\rho=\frac{5}{8 \times 10^{28} \times 1.6 \times 10^{-19} \times 2.5 \times 10^{-4} \times 0.1}$
$=1.6 \times 10^{-5} \,\Omega m$