The correct answer is option 2, the Equatorial Regions.

The Equatorial Regions experience the least variation in incoming solar radiation throughout the year. This is because the Equator is located at the center of the Earth and is perpendicular to the Sun`s rays. As a result, the Equatorial Regions receive sunlight almost directly overhead year-round, leading to a relatively constant amount of solar radiation.

On the other hand, the Poles (option 1) experience the most variation in incoming solar radiation throughout the year. This is because the Poles are located at extreme latitudes, which means that the angle of the Sun`s rays becomes more oblique during different seasons. Consequently, the amount of solar radiation received at the Poles varies significantly throughout the year.

The Tropics of Cancer and Capricorn (option 3) and the Arctic and Antarctic Circles (option 4) also experience some variation in incoming solar radiation throughout the year, but it is less pronounced compared to the Poles. However, they still experience more variation than the Equatorial Regions.

Therefore, the Equatorial Regions, being closer to the Equator and receiving sunlight almost directly overhead year-round, experience the least variation in incoming solar radiation throughout the year.