The area of the parallelogram formed by the tangents drawn at the endpoints of a pair of conjugate diameters of an ellipse is:

(a) Dependent on the specific ellipse and its diameters.
(b) Always equal to zero.
(c) Constant and equal to the product of the major and minor radii of the ellipse.
(d) Constant and equal to the square of the distance between the foci of the ellipse.