The category of nutrient best described as compounds needed in small amounts in the diet to facilitate chemical reactions in the body is vitamins.

Here's why:

* Vitamins: These organic compounds are essential for various bodily functions. They act as coenzymes, helping enzymes (biological catalysts) perform their reactions efficiently.

* Minerals: While minerals are also essential and play a role in various chemical processes, they are not specifically described as facilitators of reactions in the same way vitamins are.

* Carbohydrates, Fats, and Proteins: These are macronutrients providing energy and building blocks for the body, but they are not primarily involved in directly facilitating chemical reactions.

Therefore, vitamins are the most accurate answer to your question.