d
\(\because \mathrm{R}=\frac{\rho \ell}{A}=\frac{\rho \mathrm{V}}{\mathrm{A}^2}\)

\(\therefore \frac{\mathrm{R}_{\mathrm{A}}}{\mathrm{R}_{\mathrm{B}}}=\frac{\mathrm{A}_{\mathrm{B}}^2}{\mathrm{~A}_{\mathrm{A}}^2}=\frac{\mathrm{r}_{\mathrm{B}}^4}{\mathrm{r}_{\mathrm{A}}^4}\)

\(\Rightarrow \frac{\mathrm{R}_{\mathrm{A}}}{2}=\left[\frac{4 \times 10^{-3}}{2 \times 10^{-3}}\right]^4\)

\(\Rightarrow \mathrm{R}_{\mathrm{A}}=32 \Omega .\)