Final answer:
Newton's first law of motion is known as the Law of Inertia, which describes an object's resistance to changes in its motion and is directly related to its mass.
Step-by-step explanation:
Newton's first law of motion is also known as the Law of Inertia. This law states that an object will remain at rest, or in uniform motion in a straight line, unless acted upon by a net external force. Inertia is the property that quantifies how much resistance an object has to any change in its state of motion, and it is directly related to the mass of the object. While momentum is a measure of the motion of an object and depends on both its mass and its velocity, it is distinct from inertia, which does not involve velocity. Therefore, the correct answer to the given question is B. Inertia.