Final answer:
The reaction force to Billy's weight pushing down on the chair is the chair pushing back up on Billy, according to Newton's Third Law of Motion.
Step-by-step explanation:
When Billy is sitting in a chair in class, the action force is indeed the downward push of his weight on the chair. According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction. Therefore, the reaction force to Billy's weight pushing down on the chair would be the chair pushing back up on Billy with an equal force. This is to say, the upward force of the chair on Billy is what keeps him from falling through the chair. Hence, the correct answer is (B) the upward force of the chair pushing on Billy.