Final answer:
The upward force on a person in a downward accelerating elevator must be less than the person's weight, which is equal to 750 N, due to Newton's second law of motion.
Step-by-step explanation:
The question asks what the upward force must be on a person standing in an elevator that is accelerating downward. According to Newton's second law of motion Fnet = m × a, where Fnet is the net force on the person, m is the mass, and a is the acceleration. When the elevator is accelerating downward, the net force on the person will be less than the gravitational force due to the person's weight because the direction of the acceleration is opposite to the upward force exerted by the elevator floor. Thus, the force exerted by the elevator floor (upward force) must be less than 750 N.