All the points equidistant to a given point would form a. an ellipse b. a line c. a parabola d. a circle

For this case we have to make some definitions:
 Equidistant points: points that have the same distance from a given point.
 We observed that all the points in a circle are equidistant to the point in common that is the center of the circle.
 The distance between the points and the center of the circle is the radius of the circle.
 
 All the points equidistant to a given point would form:
 D. a circle

Definitely