Correct Answer : The point of sky overhead the observer, on the celestial sphere
Explanation : Zenith is defined as the point of sky overhead the observer, on the celestial sphere. It can be also described as the intersection point of the celestial sphere & the normal drawn to the celestial horizon. As the celestial horizon depends on the position of the observer on the Earth, it can coincide with the Celestial North Pole & Celestial South Pole, but that is not always the case. Zenith can never be the centre of the celestial sphere because it does not lie on the sphere boundary.
