What does the modern definition of inertia state?

The modern definition of inertia is closely associated with Newton's First Law of Motion, which states that an object will continue moving in a straight line at a constant velocity unless acted upon by a net external force. This reflects the concept that inertia is the tendency of an object to resist changes in its state of motion.

Inertia demonstrates that an object at rest will indeed remain at rest and an object in motion will maintain its trajectory unless a force disrupts that motion. Therefore, the statement that an object will continue in a straight line unless acted upon captures this principle effectively.

While the other statements touch on concepts related to motion, they do not encompass the full definition of inertia. An object at rest staying at rest is a subset of inertia, but it's not the complete definition. The notions of acceleration and changing direction under applied forces pertain to different fundamental concepts of motion rather than inertia specifically.