Mass is the amount of matter in an object whereas weight is the force of gravity acting on the mass of an object. Different planets exert a different force of gravity on an object-meaning that an object's weight will change depending on the force of gravity acting on it, but it's mad will remain unchanged.
When a swimmer pushes through water to swim they are propelled forward because of the water resistance against the hand and feet. It's A. The water doesn't automatically push the swimmer forward. It releases a reaction after the swimmer pushes through the water.