Final answer:
The reaction of the ground to Joan pushing with her legs is to push her upwards, according to Newton's third law of motion. The correct answer is option C.
Step-by-step explanation:
When Joan pushes the ground with her legs, the ground's reaction is to push her upwards. This is an example of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. If Joan applies a force to the ground by pushing down with her legs, the ground applies a force of equal magnitude but in the opposite direction, pushing her upward. This principle is consistent with situations such as a jumper pushing down on the ground and the ground pushing up on the jumper, as well as a rocket pushing on gas and the gas pushing back on the rocket. The correct answer is therefore option C: It pushes her upwards.