Newton's Laws - Mission NL2 Detailed Help

An object moving at a constant velocity MUST ...

Newton's First Law of Motion:
An object at rest will stay at rest; an object in motion will stay in motion with the same speed and direction, unless acted upon by an unbalanced force.

Forces when unbalanced cause objects to accelerate. So if an object is moving at a constant velocity, there must NOT be an unbalanced force. If there is no unbalanced force, then the forces acting upon it are balanced.