The albedo of earth's surface is only about 4%, yet the combined albedo of earth and its atmosphere is about 30%. which set of conditions below best explains why this is so?

For the answer to the question above, I believe the answer to these is high albedo of clouds, low albedo of water
Clouds have higher albedo because of its color which is white, White clouds deflect more sunlight and UV back to the outer space than the ocean which absorbs the heat of the sunlight

Out of the given sets of conditions, the one that best explains why is the albedo of the Earth's surface less than that of the atmosphere is - High albedo of clouds, low albedo of water.

Explanation:

Albedo is the capacity of a material to reflect light. A material with high albedo reflects more light and the one with low albedo reflects less light. Now, as we know, most of the 'Earth's surface' is covered with 'water' and most of the atmosphere is characterized by the abundance of clouds. Water absorbs more light than it reflects and the clouds 'reflect more light' than they absorb. That is clearly why the 'combined albedo' of the Earth and the atmosphere is more than the Earth alone.