The best argument that supports the student's claim that box 1 must have a smaller mass than box 2 when the same force is applied to both is:
Heavier objects need greater force than lighter objects to accelerate at the same rate.
This statement aligns with Newton's second law of motion, which states that force equals mass times acceleration (F = ma). If both boxes are experiencing the same force and box 1 accelerates more than box 2, then box 1 must have a smaller mass than box 2 in order for that to occur under the same force.