Final answer:
The greatest change in an object's speed occurs when a large net force is applied to a light object, as this induces the greatest acceleration according to Newton's second law.
Step-by-step explanation:
The condition that would cause the greatest change in an object's speed is (C) a large net force applied to a light object. According to Newton's second law of motion, the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass (a = F/m). Therefore, a large net force applied to a light object would result in the greatest acceleration, and thus the greatest change in speed. For example, if you apply the same force to a tennis ball and a bowling ball, the tennis ball, being much lighter, will accelerate much more and hence, its speed will change more compared to the bowling ball.