b
\(J=\frac{I}{A}\)

\(I=J A\)

\(=4 \times 10^{6} \times\left[\pi R^{2}-\pi\left(\frac{R}{2}\right)^{2}\right]\)

\(=4 \times 10^{6} \times \pi R^{2} \times \frac{3}{4}\)

\(=4 \times 10^{6} \times \pi \times\left(4 \times 10^{-3}\right)^{2} \times \frac{3}{4}=48 \; \pi A .\)