Final answer:
With balanced forces, an object must have a constant velocity, meaning it is either at rest or moving at a constant speed in a straight line. The object does not accelerate since there is no net external force applied.
Step-by-step explanation:
If the forces acting on an object are balanced, the object must have a constant velocity. This could mean that the object is either at rest (not moving at all) or moving at a constant speed in a straight line. According to Newton's first law of motion, an object in a state of uniform motion remains in that state unless acted upon by a net external force. Therefore, if the forces are balanced, there is no net external force, and thus no change in the object's velocity. There is no acceleration since acceleration is the rate of change of velocity.
Looking at the options provided in the question, the correct answer is D. It must have a constant velocity. Options A, B, and C suggest a change in the state of motion, which would require an unbalanced force resulting in acceleration.