The answer is choice D. All coplanar points equidistant from a given point.
In other words, all points that arethe same distance away from a fixed point. For example, if you have some fence post and you are 2 feet away from the fencepost, then that marks one point on the circle. Then move to another point that is also 2 feet away from the fencepost. Repeat until you have a bunch of points. All of these points will form a circle. The term "coplanar" means "all in the same plane". A plane is a flat surface like a flat piece of paper that doesn't bend or curve.