Terrestrial coordinates expanded into infinite space onto a celestial sphere is called the A) ecliptic plane
B) galactic plane
C) equatorial coordinate system
D) terrestrial coordinate system

The correct option is;

C) Equatorial coordinate system

Explanation:

The equatorial coordinate system is which has the most wide spread use in coordinate system for astronomy for mapping the location of celestial bodies such as stars by use of an imaginary projected celestial sphere or to rectangular coordinates with the Earth at the center.  Extending the Earth's axis onto the celestial sphere is essentially the projection of the Earths axis outwards to intersect the sphere at the celestial poles.