The correct answer is C) Horizon. In the horizon system of coordinates, the equivalent to the celestial equator in the celestial equator system is the horizon. The horizon is the great circle that divides the celestial sphere into the upper and lower hemispheres, and it is perpendicular to the vertical line passing through the observer's zenith and nadir. The other options are incorrect because: A) The prime meridian is the reference line for longitude in the celestial equator system, not the horizon system. B) The prime vertical circle is a different great circle in the celestial equator system, not the equivalent to the celestial equator. D) The principal vertical circle is also a different great circle, not the equivalent to the celestial equator.