Final answer:
Newton's first law of motion, or the law of inertia, is only valid in the absence of a net force. This law states that an object will continue in its current state of motion unless a net force acts upon it. Therefore, the answer is Net force.
Step-by-step explanation:
Newton's first law, also known as the law of inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction unless acted upon by a net force. So, Newton's first law is valid only in the absence of a net force. A net force is the vector sum of all the forces acting on an object. If there is no net force acting on an object, it will maintain its state of motion. This state of motion could be either at rest or moving at a constant velocity. Therefore, the answer to your question is b) Net force. Friction is an external force that can affect the motion of objects and is not accounted for in Newton's first law. Friction can cause objects to slow down or stop, which is why Newton's first law is valid only in the absence of friction.
Learn more about Newton's First Law