* Newton's First Law of Motion (Law of Inertia): An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

* Balanced Forces: When forces are balanced, they cancel each other out. This means there is no net force acting on the object.

* Constant Velocity: If there's no net force, the object will either remain at rest (zero velocity) or continue moving at a constant velocity (same speed and direction).

Let's eliminate the other options:

* A - must not be moving: This is incorrect because the object could be moving at a constant velocity.

* C - must be accelerating: This is incorrect because acceleration requires an unbalanced force.

Therefore, if the forces acting upon an object are balanced, the object must be moving with constant velocity.