The correct answer is C) Celestial Meridian. The celestial meridian is always needed to form the astronomical triangle, which is the spherical triangle formed by the celestial pole, the celestial object, and the zenith. This is because the celestial meridian is the great circle that passes through the celestial pole and the zenith, and it intersects the celestial object's position, defining one side of the astronomical triangle. The other options are incorrect because the prime vertical circle, prime meridian, and celestial equator are not necessary to form the astronomical triangle. They are other important reference circles in celestial navigation, but not specifically required for the astronomical triangle.