Final answer:
Inertia is the correct term for the tendency of an object to continue in its state of rest or motion unless acted upon by an external force, as outlined in Newton's first law of motion. It is directly related to mass.
Step-by-step explanation:
The tendency of a moving object to remain in unchanging motion in the absence of an unbalanced force is called inertia. Newtons first law is often referred to as the law of inertia and it states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted upon by a net external force. Inertia is directly related to an object's mass, which is the quantity of matter in a substance.
In practical terms, it is more difficult to change the motion of objects with greater mass, like a large boulder, compared to that of smaller mass, like a basketball. Therefore, the correct answer to the question is D. inertia.