The best argument to support the student's claim that box 1 must have a smaller mass than box 2 is:
"Heavier objects need greater force than lighter objects to accelerate at the same rate."
In this scenario, both boxes are subjected to the same force (40 N). Since box 1 accelerates at a higher rate (10 m/s²) compared to box 2 (5 m/s²), it indicates that box 1 has a smaller mass. According to Newton's second law of motion (F = ma), for a given force, if the acceleration is greater, the mass must be lesser, supporting the student's claim.