The best argument to support the student's claim that box 1 (which experiences an acceleration of 10 m/s²) must have a smaller mass than box 2 (which experiences an acceleration of 5 m/s²) is:
"Heavier objects need greater force than lighter objects to accelerate at the same rate."
This is consistent with Newton's second law of motion (F = m*a), which states that for a given force, an object with a larger mass will have a smaller acceleration. Since both boxes are subjected to the same force (40 N), the box that experiences greater acceleration (box 1) must indeed have a smaller mass than the box that experiences lesser acceleration (box 2).