Answer
They all have the same speed at the bottom. In case C the most work is done on the mass by the time it reaches the bottom.
Work Step by Step
Mechanical energy is conserved in the absence of air resistance and friction. The speed at the bottom is $v=\sqrt{2gh}$ for all of them.
The work done by gravity is the change in kinetic energy, by the work-energy theorem. The large mass in case C has the greatest kinetic energy at the bottom.
