When the ball's velocity becomes constant, it has reached its terminal velocity, a state where the net force acting on the object is zero , . In this equilibrium state, the downward force of gravity (weight, $W$) is perfectly balanced by the sum of the upward forces: the buoyant force ($B$) and the viscous force ($F_v$) .

1. Weight ($W$): Given as $W = Mg$.
2. Buoyant Force ($B$): This force is equal to the weight of the displaced fluid. Since the ball has mass $M$ and density $d$, its volume is $V = M/d$ . The density of glycerine is given as $d/2$. Therefore, $B = V \times (d/2) \times g = (M/d) \times (d/2) \times g = Mg/2$ .
3. Equilibrium Equation: At terminal velocity, $F_v + B = W$. Substituting the values, we get $F_v + Mg/2 = Mg$.
4. Result: Solving for the viscous force, $F_v = Mg - Mg/2 = Mg/2$.