b
Let length of submerged part $=\mathrm{h}$
then $\mathrm{Ah} \rho=\mathrm{A} \ell \rho_{0}$
$\Rightarrow \mathrm{h}=\frac{\rho_{0} \ell}{\rho}$
$\therefore \mathrm{T}=2 \pi \sqrt{\frac{\mathrm{h}}{\mathrm{g}}} \Rightarrow \mathrm{T}=2 \pi \sqrt{\frac{\rho_{0} \ell}{\rho \mathrm{g}}}$