Which of the following would most likely be considered nonpoint source pollution a. animal waste from a local horse farm seeping into the ground b. increased turbidity from erosion c. a car repair shop dumping used oil in the ground d. a leaky underground oil tank

The best answer is b) increased turbidity from erosion.

Nonpoint source pollution generally happens as a result of many systems interacting, and is not directly attributed to one event or pollutant. Generally, natural environmental systems participate in pollution of this kind, regardless of whether or not human activity was a factor. Examples include water runoff, or erosion. 

The other pollutants listed have a direct cause and direct effect, the animal waste goes directly from the animals to the ground they live on, the car shop directly sumps the oil on the ground, and the oil tank leaks directly into the earth. Erosion causing turbidity is a less direct form of pollution, and is due to the synthesis of several natural phenomena