Final answer:
The reaction force to the weight of the cat acting downward is the upward normal force exerted by the table on the cat, in accordance with Newton's Third Law of Motion.
Step-by-step explanation:
The question pertains to Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. When a cat sits at rest on a table, the weight of the cat exerts a downward force due to gravity on the table. According to Newton's Third Law, the table must exert an equal and opposite upward force on the cat. This reaction force is known as the normal force, and it is the force that prevents the cat from falling through the table. Consequently, the Newton's Third Law reaction force to the weight of the cat acting downward is the upward normal force exerted by the table on the cat.