Final answer:
Most of the visible light headed toward Earth on a cloudless day reaches the Earth's surface.
Step-by-step explanation:
On a cloudless day, most of the visible light headed toward Earth (d) reaches the Earth's surface. When light from the Sun reaches the Earth's upper atmosphere, it can travel through empty space directly or interact with objects like air or glass. In a cloudless sky, there are no clouds to reflect or absorb the visible light, so it continues its path and reaches the Earth's surface.