Answer:
B. the tilt of earths axis
Explanation:
The orientation of the tilt of earths axis through the year is (almost) the same relative to the stars far away. As the Earth is tilted, the Equator in the side of the sun is over the orbital plane in one side of the orbit and under the orbital plane in the other side.
As the earth travel from one point of the orbit to the other, every day the sun appears to move through the sky at different altitude.
I attached an sketch, hope its helpful.