Since the ball attains equilibrium when the velocity becomes constant.
The effective force of the liquid must balance the viscous force
Effective force $= V (d_1 -d_2)g$
But $v=\large\frac{M}{d_1}$
$\therefore$ effective viscous force $= \large\frac{M}{d_1}$$(d_1-d_2)g$
$ \qquad= Mg (1- \large\frac{d_2}{d_1})$
Hence a is the correct answer.