What is inertia?

Inertia is defined as the property of matter that causes it to resist changes in its state of motion. This means that an object will either remain at rest or continue moving at a constant velocity unless acted upon by an external force. This characteristic is tied to Newton's first law of motion, which states that an object will not change its motion unless a force is applied. Therefore, the correct understanding of inertia highlights its role in maintaining the status quo of motion, either at rest or in uniform motion, which aligns perfectly with the chosen answer.

In contrast, the other options do not accurately define inertia. Changing an object's shape relates to elasticity, while the measurement of force pertains to dynamics. The amount of speed of an object refers to its velocity, not to the concept of inertia and how it resists changes to that motion. Hence, the chosen answer encapsulates the fundamental principle of inertia accurately.