Solar radiation per unit area received at the outer limit of the atmosphere is known as:

Correct option is C

The correct answer is Solar constant.
Solar Constant:
· Solar constant is the total radiation energy received from the Sun per unit time per unit area on a theoretical surface perpendicular to the Sun's rays at Earth's mean distance from the Sun.
· It is measured by satellites where atmospheric effects are absent.
· The value of the constant is approximately 1.366 kilowatts per square meter (kW/m²).
· Though called a constant, the solar constant is relatively constant rather than absolutely fixed.
· The relative constant varies by 0.2% in a cycle that peaks once every eleven years.
· The first attempt at estimating the solar constant was made by Claude Pouillet in 1838 at 1.228 kW/m².
Thus, we can say that the solar constant is the quantity of solar radiation per unit area received at the outer limit of the atmosphere.
Additional Information
· Insolation is a measure of solar energy that is incident on a specified area over a set period of time.
· It affects temperature: The higher the insolation, the higher the temperature.
· Heat budget refers to the perfect balance between incoming heat absorbed by Earth and outgoing heat escaping as radiation.
· If this balance is disturbed, the Earth would get progressively warmer or cooler with each passing year.
· Solar energy is the radiation from the Sun, capable of producing heat, causing chemical reactions, or generating electricity.
· It is harnessed using a range of technologies, such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture.