The best explanation for the advantage of using the ramp instead of lifting the speaker directly onto the stage is:
C) less force is required by using the ramp.
Using a ramp allows the students to spread the required lifting force over a longer distance, which reduces the amount of force they need to exert at any one time. While the total work done (force x distance) remains the same, the ramp makes it easier to move the speaker by requiring less force to lift it.