It is the measure of the inertia of a body.

The correct answer is: Mass

The property of a body by virtue of which it tends to retain its existing state of rest or its uniform motion along a straight line unless and until an external force is applied on it is known as Inertia.
Inertia is mainly of three types:
1. Inertia of rest: When a bus or train starts to move suddenly after being in rest, a jerk is experienced by the passengers sitting inside and they fall in a backward direction due to inertia of rest.
2. Inertia of motion: When a bus or train stops suddenly after being in motion, a jerk is experienced by the passengers sitting inside and they lean in the forward direction due to inertia of motion.
3. Inertia of direction: We can use an umbrella to protect ourselves from rain as the rain drops falls downwards and doesn’t change its direction due to inertia of direction.
Inertia is a measure of mass of a body and it is directly proportional to mass of a body. This means greater the mass of a body greater will be its inertia or vice-versa. A more heavy body will have a greater tendency to retain its inertia of rest or motion.
Therefore, mass of the body is the measure of inertia.