Final answer:
Net radiation at the thermopause is positive, indicating that it is a source of heat for Earth, helping to maintain a relatively constant temperature suitable for life through the balance of incoming solar energy and outgoing infrared radiation and the greenhouse effect. So, the option is A.
Step-by-step explanation:
The Earth is in approximate thermodynamic equilibrium, where the incoming solar energy is balanced by the outgoing infrared radiation. This results in a steady net energy flow that maintains the Earth's relatively constant temperature, providing conditions suitable for life.
The Stefan-Boltzmann law describes the power radiated from a body based on the difference in temperature between the radiating object and its surroundings and is applied at the thermopause.
Moreover, due to the greenhouse effect, certain gases like carbon dioxide (CO₂) and water (H₂O) absorb most of the outgoing infrared radiation from Earth and re-radiated it, which contributes to a positive net radiation balance at the thermopause.
Therefore, A is the correct option.