To support the student's claim that box 1 must have a smaller mass than box 2, the best argument from the options provided is:
B. Heavier objects require greater force than lighter objects to accelerate at the same rate.
This statement aligns with Newton's second law of motion (F = ma), which indicates that for a given force, acceleration is inversely proportional to mass. Since both boxes are experiencing the same force (20 newtons), the box that accelerates more (box 1) must indeed have a smaller mass than the box that accelerates less (box 2). Thus, the supporting reasoning aligns with the student's conclusion.