The argument that best supports the student's claim is:
"Heavier objects need greater force than lighter objects to accelerate at the same rate."
This statement implies that for a given force (in this case, 40 N), a lighter object (box 1) will experience a greater acceleration than a heavier object (box 2), which aligns with the observed accelerations of 10 m/s² for box 1 and 5 m/s² for box 2. Therefore, since box 1 accelerates more than box 2 under the same force, box 1 must have a smaller mass.