Concept:The solar constant is the amount of solar radiation received per unit area at the top of Earth's atmosphere, at the average Earth-Sun distance.
Explanation:The question asks for the term that defines solar radiation per unit area at the outer limit of the atmosphere.
The solar constant is measured by satellites above the atmosphere, where no interference occurs.
Its approximate value is 1.366 kW/m².
Even though it is called a constant, it varies slightly by about 0.2% over an 11‑year cycle.
The other options do not match this definition.
Insolation refers to solar energy received on a given area over a specific time period on the Earth's surface, not at the outer limit.
Heat budget describes the balance between incoming absorbed radiation and outgoing radiation from Earth.
Solar energy is the broad term for radiation from the Sun that can be converted into heat or electricity.
Thus, only the solar constant correctly describes the radiation received per unit area at the atmosphere's outer boundary.
Answer:C. Solar constant