How difficult it is to change an objects motion. Think of a tablecloth; it takes energy to move it from underneath objects on the table, and it takes energy to move the objects themselves on the table.
Sorry I'm late. Here is an example: I have two equal bricks, with the difference of material. One is foam and the other is cement. If I give both an equal push which one would be easier to push? The foam brick...... because it has less inertia. The more inertia the more resistance you will have, just like the cement brick. So the answer would be "How difficult it is to change an object's motion."
Answer: Increasing the volume of the gas would be the correct answer. Charles law of gas states this: 1) If the volume of a container is increased, the temperature increases.
2) If the volume of a container is decreased, the temperature decreases.