The correct answer is A) Zenith and nadir. In the horizon system of coordinates, the zenith and nadir are the points on the celestial sphere directly overhead and directly underfoot, respectively. These points are equivalent to the geographic north and south poles on the Earth's surface. The other options are incorrect because: B) Nodes refer to the points where the orbit of a celestial body intersects a reference plane, such as the ecliptic. C) Ecliptic poles are the points on the celestial sphere where the ecliptic, the apparent path of the Sun, intersects the celestial sphere. D) Celestial poles are the points on the celestial sphere where the Earth's axis of rotation intersects the celestial sphere.