The boy who ran for class president is the football captain.
The correct pronoun is who.
The pronoun which is used for objects, not people.
I think it is C because A is missing commas, B doesn't need them and D doesn't need one.
Organic matter could mean a lot of things. When we stick to the scientific definition, there are compounds that contain the elements hydrocarbons. Most of these compounds are found in living things. However, when used in this example, organic matter is used to feed living things. Thus, this means that organic matter means food.
