The sun appears to move through the background stars. This apparent motion would not exist if:. a) the moon did not orbit the earth
b) the earth did not orbit the sun
c) the earth did not turn on its axis once a day

b) the earth did not orbit the sun

Explanation:

The sun appears to move through the background stars due to parallax . Parallax is a phenomenon when the near object appears to move faster than the distant object . If we travel in a train , the near object like electric poles on rail track or trees and fields nearby appear to move faster against the distant background .

Hence when the earth moves around the sun , the sun appears to move against the background the stars which are far away . Had earth remained stationary at a place on its orbit around the sun , the sun would have appeared stationary against the background the   stars .

b.transmitters send radio waves, and receivers capture radio waves.