C. They must be obtained through diet.
Explanation - Essential nutrients are those nutrients that are not produced by the body and instead needs to be taken in by having food. Example - carbohydrates, proteins, fats, vitamins and minerals. We can get most of the nutrients if we consume balanced diet.
Non-essential nutrients are produced by the body and need to rely completely on food or any other dietary substances.
Example - cholesterol. Thus in this case, we can say that essential nutrients are consumed via diet.