The correct answer is: A. Change the motion of the body.
A retarding force is a force that opposes the motion of an object. It can cause the object to slow down, stop, or change direction.
Option B is incorrect because a retarding force does retard the motion of the body.
Option C is incorrect because a retarding force does not introduce the motion of the body. The motion of the body must already be present in order for a retarding force to act upon it.
Option D is incorrect because only option A is correct.