Final answer:
At a 90° angle of incidence, around 70% of solar radiation reaches the Earth's surface, with the rest being reflected, so the correct answer is closest to 75%.
Step-by-step explanation:
When the sun's angle of incidence is 90°, not 100% of the radiation actually reaches the surface of Earth. While a 90° angle of incidence (meaning the sun's rays are hitting the Earth's surface directly overhead) is the most efficient angle for sunlight to reach the Earth's surface, it does not mean that all sunlight is absorbed. Approximately 30% of the incoming light is reflected by clouds, snow, and surfaces like water and terrain.
Therefore, around 70% of the incoming solar radiation is absorbed by the Earth's surface. The answer to the question is therefore closest to option b) 75%.