which macromolecule is the main source of energy for living things?\no fat\no nucleic acid\no protein\no…

Brief Explanations:

Carbohydrates are broken down into glucose which is used as the main energy - source for cells in living organisms. Fats are energy - storage molecules, nucleic acids store genetic information, and proteins have various functions like structure and catalysis but are not the main energy source.

Answer:

D. carbohydrate