The correct answer is - C. solar radiation.
The climate is very complex and it is influenced by lots of different factors, but the biggest of them all is the solar radiation, and it is directly influencing the change of seasons.
When the solar radiation comes at a more direct angle, the heat is more concentrated, and thus heats up the place more, and that is when the summer appears, but when the solar radiation is coming at a sharper angle, than the heat is more dispersed, thus heating up the place much less, and that is when the winter appears. Between these two extremes, are the spring and autumn, when the solar radiation falls at an angle that is neither close to direct, nor very sharp, thus the climate is mild.
