Aconstellation is that forms a pattern resembling animals, mythological characters, or objects.
a. a group of meteors
b. a group of stars
c. an entire galaxy
d. one single planet

The answer is the option B. a a group of stars. So the complete sentence is "A constellation is a group of stars that forms a pattern resembling animals, mythological characters, or objects. Most names of constellations originated in ancient times and took their names from the patterns that observer could watch in the sky. Some examples of constellations are Orion, Aquarium, Cancer, etc.

What formula  this one  (x_/