1. You only can answer this question if you know the density of the fluid. I assume that you use water as the fluid which has a density of
2. The difference of the weights is caused by the buoyant force of the cork. The buoayant force is as great as the weight of the displaced fluid.
3. Obviously the weight of the displaced fluid is 100 cN. Assuming that the fluid is water then the volume of the displaced fluid is 100 cm³ - and that must be the volume of the cork too.