Let's determine what an object's natural resistance to motion is called.
The natural tendency of an object to resist changes in their state of motion can be said to be INERTIA.
Inertia can be said to be a passive property which only enables an object to oppose motion.
Inertia can also be said to be the tendency of an object to remain in motion, or an object to remain at rest unless acted upon by an external force.
Therefore, an object's natural resistance to motion is called inertia.
ANSWER:
Inertia