From Newton's First Law of Motion, we know that if an object does not experience any force, it will maintain its movement state, either at rest or moving with a constant speed and constant direction.
From Newton's Second Law of Motion, we know that the net force acting on an object equals the product of the mass of the object and its acceleration:
If the forces acting upon an object are balanced, then:
Which implies that the acceleration of the object is 0:
The given options b and c can be correct but they do not represent the most general case.
For option b), an object can be at rest if it does not accelerate but it could also be moving with constant velocity and its acceleration would be 0.
For option c), an object with constant speed could be changing the direction of its speed and it would be accelerating, as a satellite orbiting a planet in a circular trajectory.
The phrase that correctly describes all the cases in which an object can move if the forces are balanced, is: the object is not accelerating.
Therefore, the correct choice is Option D) Not accelerating.