Group of stars:

Answer: Option B

The one word substitution is Constellation.

Orbit : an area of activity, interest, or influence.
Solar system : the collection of eight planets and their moons in orbit round the sun, together with smaller bodies in the form of asteroids, meteoroids, and comets.
Comet : a celestial object consisting of a nucleus of ice and dust and, when near the sun, a 'tail' of gas and dust particles pointing away from the sun.
Constellation : Group of stars.