a
\(\rho_{B}=2 \rho_{A}\)

\(d_{B}=2 d_{A}\)

\(R_{B}=R_{A} \Rightarrow \frac{\rho_{B} \ell_{B}}{A_{B}}=\frac{\rho_{A} \ell_{A}}{A_{A}}\)

\(\therefore \frac{\ell_{B}}{\ell_{A}}=\frac{\rho_{A}}{\rho_{B}} \times \frac{d_{B}^{2}}{d_{A}^{2}}=\frac{\rho_{A}}{2 \rho_{A}} \times \frac{4 d_{A}^{2}}{d_{A}^{2}}=2\)