Answer: The Earth's tilt.
Explanation:
The reason why the Earth has seasons is because our planet’s axis of rotation is tilted at an angle of 23.5 degrees relative to our orbital plane. This tilt in the axis is called obliquity.
During a yeartime, there is no variation in the angle of tilt. However, the orientation of Earth’s tilt with respect to the sun changes as we orbit the sun. This change causes the different seasons.