What causes inertia?

Hint: The inertia is the tendency of all the objects/bodies to resist the change in their motion. The inertia is directly proportional to the mass of the object/body, that is, the greater the mass of the body/object, the greater will be the inertia possessed by that object/body.

Complete answer:
The tendency of all the objects/bodies to resist the change in their motion is called inertia. That is, an object/body at rest will stay at rest until and unless a force causes it to move. Similarly, an object/body in motion will stay in motion until and unless a force causes it to stop or change its speed.
Thus, as per the definition of inertia, it is caused by the resistance which is offered by the body/object to change in its state of rest or motion.
The inertia is directly proportional to the mass of the object/body, that is, the greater the mass of the body/object, the greater will be the inertia possessed by that object/body.
Newton’s first law of motion and/or Galileo’s law of inertia define this property, as these laws deal with the property of the body/object to resist any change in its state of rest or of motion.
\[\therefore \]The cause of inertia is the resistance offered by the body/object to change in its state of rest or motion.

Note:
Newton’s first law of motion states that a body at rest or in motion will remain in that state until and unless it is compelled by an external force. Galileo’s law of inertia states that a body in motion would remain in that state unless a force (maybe friction) caused it to come to rest.