Final answer:
The reaction force to Earth's gravity when a diver jumps off a diving board is the diver pulling Earth up, in accordance with Newton's third law of motion.
Step-by-step explanation:
When a diver jumps off a diving board, gravity pulls him or her down to the water. The reaction force to Earth's gravity, according to Newton's third law of motion, is the diver pulling Earth up. This mutual force means that both the diver and Earth experience the same total change in momentum. However, since Earth has a significantly greater mass than the diver, its acceleration is minuscule and cannot be measured. Thus, the correct answer is 4) The diver pulls Earth up, though the movement of Earth is imperceptible to us.