Final answer:
Option C. Lipids yield 9 kcal/g when used for energy and they provide more than double the energy density compared to carbohydrates and proteins, which each offer 4 kcal/g.
Step-by-step explanation:
The type of nutrient that yields 9 kcal/g when used for energy is lipids. Lipids, also known as fats and oils, have the highest energy density among the macronutrients in our food. While carbohydrates and proteins each yield 4 kcal/g, lipids provide more than double this amount.
Macronutrients are essential nutrients that are needed in large quantities by the body, and lipids also play vital roles besides providing energy, such as protecting neurons and composing cell membranes.