Final answer:
Arcturus is the star closest to the celestial equator among the given options, as it has the smallest absolute declination value of +19°27'.
Step-by-step explanation:
The star that lies closest to the celestial equator on the celestial sphere from the options given is Arcturus, with a declination of +19°27'. The celestial equator is the projection of the Earth's equator onto the celestial sphere, and a star's declination is a measure of how far north or south the star is from the celestial equator.
The declination is expressed in degrees, with positive values indicating north and negative values indicating south. Stars with a declination close to 0° are closest to the celestial equator. Therefore, among the options provided, Arcturus (D) with the smallest absolute value for declination is the correct answer.