An example of kinetic energy continuously being changed to potential energy and back again might be
a. a girl swinging on a swing*
b. a train moving down a track
c. a plate sitting on the edge of a table
d. electric charges moving in a straight path

The correct option is A.
Kinetic energy is defined as the energy in motion and it is found in moving objects. Potential energy is defined as the energy at rest and it is found in objects that are at rest.
A swing has two phases, the resting position before the swing started moving and the working position which occurs when the swing is moving. At the resting position the swing possess potential energy and at the moving position it possesses kinetic energy. Thus, a swing alternates between potential and kinetic energy.