Final answer:
Galileo used balls of the same size but different masses to demonstrate the universality of free fall. Option 1 is correct, as his experiment aimed to prove that all objects accelerate at the same rate due to gravity, regardless of mass.
Step-by-step explanation:
Galileo conducted an experiment to investigate the effects of gravity on objects of different masses. By keeping the balls the same size but changing their masses, Galileo aimed to demonstrate the universality of free fall, meaning all objects fall at the same rate regardless of their mass when no other forces, like air resistance, are at play. The correct answer to the student's question is Option 1: To demonstrate the universality of free fall. Since Galileo's time, this principle has been foundational in physics, leading to Newton's law of gravitation, which describes the force causing objects to fall as a universally existing force of attraction between masses.