The correct answer is (a) v1 = (8Bg (G-1) d/f) ^1/2

For explanation: Displacement velocity is given by v1 = (8Bg (G-1) d/f)^1/2 where B is a constant with value range 0.04-0.06, G is specific gravity, d is the diameter of the particle and f is the Darcy Weisbach friction factor.