a
For constant velocity $F_{\text {net }}=0$
$\mathrm{F}_{\mathrm{vis}}+\rho_0 \mathrm{vg}=\rho \mathrm{vg}$
$\mathrm{F}_{\mathrm{vis}}=\left(\rho-\rho_0\right) \mathrm{vg}$
$=\rho \mathrm{vg}\left(1-\frac{\rho_0}{\rho}\right)$
$=\mathrm{Mg}\left(1-\frac{\rho_0}{\rho}\right)$
