The correct answer is C) less contaminants would flow into the drainage basin.
Explanation:
In the agricultural field, farmers use many types of pesticides, insecticides, urea and other chemical that drained off during precipitation into the drainage basin and ultimately goes into the river and other water bodies which results in eutrophication of water bodies.
So if an agricultural field is converted back into a natural prairie than the flow of the contamination into the drainage basin will become low because now no chemical will be used to maintain prairie. Therefore the correct answer is C.