The correct answer is B) celestial equator. A time diagram is a graphical representation of the relationship between time and celestial bodies, such as the sun, moon, and stars. This diagram is plotted on the plane of the celestial equator, which is the great circle on the celestial sphere that corresponds to the Earth's equator projected outward into space. The other options are incorrect because they do not represent the plane on which a time diagram is constructed. The principal vertical circle, celestial horizon, and celestial meridian are other important celestial reference planes, but they are not the primary plane used for a time diagram.