The correct answer is A) Horizon. In the horizon system of coordinates, the horizon is equivalent to the equator on the Earth. The horizon is the great circle on the celestial sphere that intersects the observer's zenith and nadir, and it divides the celestial sphere into the upper and lower hemispheres. Just as the equator is the great circle that divides the Earth into the northern and southern hemispheres, the horizon serves a similar function in the horizon system of coordinates. The other options are incorrect because they represent different components of the horizon system, such as the prime vertical circle (B), parallels of altitude (C), and the principal vertical circle (D), which are not equivalent to the equator.