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

\(\rho=\frac{5}{8 \times 10^{28} \times 1.6 \times 10^{-19} \times 2.5 \times 10^{-4} \times 0.1}\)

\(=1.6 \times 10^{-5}\, \Omega m\)