Which defines a circle? two rays with a common endpointa piece of a line with two endpointsa piece of a line with one endpointall coplanar points equidistant from a given point?

All coplanar points equidistant from a given point.

 To understand this definition you must first know that they are coplanar points.
 In geometry, a set of points in space is coplanar if all points are in the same plane.
 We have then that the coplanar points equidistant (same distance) of a given point (center of the circle), forms a circle.

 
 all coplanar points equidistant from a given point.