The source of electromagnetic waves can be a charge

We know that electromagnetic waves can be produced by a charge when, the it experiences acceleration as when oscillating. Thus when moving in a circular orbit the direction of its velocity is continuously changing and thus the charge accelerates and produces electromagnetic waves. When a charge is falling through an electric field the charge experiences a force and thus acceleration, and therefore produces electromagnetic waves. In the option (a) the charge is moving at constant velocity and has no acceleration and in (c) is at rest.