Final answer:
Soil filters groundwater as it passes through, removing contaminants and replenishing aquifers, making 'b. filter groundwater' the correct answer.
Step-by-step explanation:
Soil serves many functions related to groundwater. It acts as a natural filter that can contain and filter groundwater. As water moves through the soil, contaminants are removed, and clean water replenishes aquifers or becomes part of surface water systems. The correct answer to the student's question is: Soil serves to: b. filter groundwater.