What causes the motion of a body that is initially at rest?

An object is said to be at rest if its position with relation to its surroundings does not vary over time. An object is said to be in motion if its position changes in relation to its surroundings and time.

The first law of motion states that a body remains in the state of motion or the state of rest until and unless an external force is applied. Hence, the answer is force. This is also known as the law of inertia.