b
\(\frac{{R}_{1} {R}_{2}}{{R}_{1}+{R}_{2}}=3\)

\(\frac{\left(12 \times 10^{-6} \times 10^{-2}\right) \ell \times 4}{\frac{\pi(2)^{2} \times 10^{-6}} \times \frac{\left(51 \times 10^{-6} \times 10^{-2}\right) \ell \times 4}{\pi(2)^{2} \times 10^{-6}}}{\frac{63 \times 10^{-6} \times 10^{-2} \times \ell \times 4}{\pi(2)^{2} \times 10^{-6}}}\)

\(\Rightarrow \ell=97 {m}\)