The correct answer is A) apparent westerly motion of a planet with respect to stars. Retrograde motion refers to the apparent backward or westerly motion of a planet across the sky, relative to the fixed stars. This is caused by the Earth's orbital motion around the Sun, which causes the planet to appear to move in the opposite direction compared to its actual motion around the Sun. The other options describe different astronomical motions, but they do not specifically refer to the retrograde motion of a planet. Option B describes the movement of the points where the plane of the Earth's orbit (the ecliptic) intersects the plane of the Earth's equator. Option C describes the movement of the celestial north pole, and Option D describes the movement of a superior planet in its orbit around the Sun, which is not the same as retrograde motion.