The quantity that characterizes the inertia of linear motion of a body is mass.

Here's why:

* Inertia is the tendency of an object to resist changes in its motion.

* Mass is a fundamental property of matter that measures its resistance to acceleration.

The greater the mass of an object, the greater its inertia. This means it takes more force to change its velocity (either speed or direction).

Here's a simple example:

* It's much easier to push a small, lightweight object like a toy car than a large, heavy object like a refrigerator. The refrigerator has more mass and therefore more inertia, making it harder to get it moving.