Final answer:
Lipids are the class of organic molecules responsible for long-term energy storage in the body, due to their high caloric content and structural properties.
Step-by-step explanation:
The class of organic molecules that provides long-term energy storage is lipids. Lipids, which include fats and oils, have a higher caloric content than carbohydrates and are utilized by the body for long-term energy storage. This is due to their chemical structure which allows them to pack more energy per gram than carbohydrates. In fact, the oxidation of fats and oils supplies about 9 kcal of energy for every gram oxidized, compared to only 4 kcal/g for carbohydrates. The capacity for storing carbohydrates in the form of glycogen is limited, so the body stores excess energy as lipids.
Lipids serve various roles beyond energy storage; they are vital components of cell membranes, serve as insulation, and are necessary for the absorption of fat-soluble vitamins such as A, D, E, and K. Therefore, the correct answer to which class of organic molecules provides long-term energy storage is d. lipids.