During which step of the water cycle does water combine with pollutants on earth's surface? a. precipitation b. percolation c. runoff d. transpiration

The correct answer is option C, that is a runoff.  

The water from rain, snowmelt, or other sources is known as surface runoff, it flows over the land surface, and is a prime constituent of the water cycle. Runoff, which takes place on the surfaces prior to reaching a channel is also known as overland flow. When flowing of runoff takes place along the ground, it can pick up soil contaminants like pesticides, petroleum, or fertilizers, which turn into overland or discharged flow.