An object's motion changes a. when the forces acting on the object remain balanced. b. only if the object is at rest when a force acts upon it. c. only when a nonzero net force is applied to the object. d. all of these

C.

only when a nonzero net force is applied to the object.

Explanation:

We can use the process of elimination to solve this problem-

Firstly,  "A. when the forces acting on the object remain balanced.", can be eliminated because when all forces upon an object are equal, the object is at rest- no motion.

Based upon this we can also eliminate "D. All of these," as this would be, well incorrect.

That leaves "B. only if the object is at rest when a force acts upon it." While this is technically correct, it is not the full answer- yes, when there is 0 motion and a force acts upon an object, motion is changed, but this can happen in other ways, like acceleration, deceleration, etc.

This leaves "C. only when a nonzero net force is applied to the object."

This answer implies that any non-zero force applied on any object will create a change in motion, which is true, no matter how minute it is.

Therefore, the answer is "C. only when a nonzero net force is applied to the object."

the correct answer is 2 and 4