The correct answer is Clouds. Albedo is the measure of the reflectivity of a surface. Clouds, especially fresh, thick clouds, are very effective at reflecting solar radiation back into space and typically have a high albedo (ranging from 0.2 to 0.7 or even higher). Other surfaces listed have generally lower albedos: Vegetation (like forests) typically has a low albedo (around 0.1-0.2) as it absorbs sunlight for photosynthesis. Concrete surfaces have moderate albedo (around 0.1-0.3). Desert surfaces (sand) can have moderate to high albedo (around 0.2-0.4) depending on the color and composition of the sand, but generally lower than bright clouds. Fresh snow has the highest albedo among natural surfaces, but it was not an option.