Answer:
c) Earth's curvature, which presents varied angles to parallel solar rays.
Step-by-step explanation:
Insolation: It is a measure of the amount of solar radiation incident over a specified area on Earth over a certain period.
This helps in studying the heat or temperature of a particular region and its variation in different seasons among other things. The distribution of insolation is uneven because of Earth's curvature.
The Sun is huge and at a very large distance from Earth, so it is considered that all the rays coming from the Sun are parallel. But because of the curvature of the Earth, the incident angle of these rays will be different at different latitudes. Also, the incident angle will keep on changing throughout the year due to the Tilt of Earth and its revolution around the Sun thus resulting in seasons.