Correct Answer - C
Let r be radius of drop, `:. V=(4)/(3)pi r^(3)`
When velocity of ball becomes constant, then forces are balance. So
Viscous force, `F=(4)/(3)pi r^(3) rho g - (4)/(3)pi r^(3) sigma g`
`=(4)/(3)pi r^(3) rho g(1-(sigma)/(rho))`
`=V rho g(1-(sigma)/(rho))` `[.: Vxxrho =m]`
`=mg(1-(sigma)/(rho))`