An object's motion changes A.
when the forces acting on the object remain balanced.
B.
only when a nonzero net force is applied to the object.
C.
only if the object is at rest when a force acts upon it.
D.
when there are no forces acting upon it.

The correct answer is:
B. only when a nonzero net force is applied to the object.

Explanation:
A net force is the sum of the unbalanced forces acting on an object. An object's motion changes only when a nonzero net force is applied to the object. When a nonzero net force is applied to an object, the object's speed and/or direction changes.

If the forces acting on the object are balanced, they will cancel each other out and will not cause the motion of the object to change.