Final answer:
Mass is the property that best quantifies an object's inertia.
Step-by-step explanation:
The property that best quantifies an object's inertia is its mass. Inertia is the resistance of an object to changes in its state of motion, and mass is the measure of the amount of matter in an object. Objects with greater mass have greater inertia, meaning they are more resistant to changes in their motion. For example, a heavy object requires more force to accelerate than a light object.