We use Archimides' principle, which states that the buoyant force on an object is always directed upward and its magnitude is equal to the weight of the fluid displaced. Recall that a fluid can be either a gas or a liquid. So, let denote that volume of the balloon. We neglect the volume of the suspended mass. The buoyant force is then . This must equal the weight of the helium-filled balloon plus the weight of the mass. That is
This can be easily solved for :
Therefore, answer (D) is correct.