The uneven distribution of insolation (incoming solar radiation) on the Earth is mainly due to the spherical shape of the Earth.

Option 1, the spherical shape of the Earth, leads to variations in the angle at which sunlight reaches different parts of the Earth`s surface. The angle of incidence of sunlight affects the intensity of solar radiation received, with areas near the equator receiving more direct sunlight and higher insolation, while areas closer to the poles receive sunlight at a lower angle and experience lower insolation.

Option 2, the distribution of land and water, also plays a role in the uneven distribution of insolation. Land areas heat up and cool down more quickly than water bodies, leading to variations in temperature and atmospheric pressure, which in turn influence wind patterns and the redistribution of heat.

Options 3 and 4, the direction of winds and the presence of pressure belts, are not the main factors responsible for the uneven distribution of insolation. While they do influence the movement of air masses, these factors are secondary to the shape of the Earth in determining insolation patterns.

In summary, the main reason for the uneven distribution of insolation on the Earth is the spherical shape, which leads to variations in the angle at which sunlight reaches different parts of the Earth`s surface