The best argument that supports 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.
This argument aligns with Newton's second law of motion (F = ma), which states that the force applied to an object is equal to the mass of that object multiplied by its acceleration. If the same force is applied to both boxes and box 1 accelerates, it implies that its mass must be less compared to box 2, as a heavier box (greater mass) would require a greater force to achieve the same acceleration.