The correct answer is
D.
unbalanced forces
In fact, according to Newton's second law, unbalanced forces acting on an object will accelerate the object:
\(\sum F = ma\)
where the term on the left is the resultant of the forces acting on the object, m is the object's mass, and a is its acceleration.
If there are unbalanced forces acting on the object, the acceleration of the object a is different from zero: this means that the object will change its speed or its direction, or it can also stop moving if the direction of the force is opposite to that of the motion.
