Question

which macromolecule is the main source of energy for living things?

fat
nucleic acid
protein
carbohydrate

Explanation:

Carbohydrates are the primary energy source for living organisms. They are broken down into glucose, which is used in cellular respiration to produce ATP (adenosine triphosphate), the energy currency of the cell. Proteins are mainly for growth, repair, and as enzymes/hormones. Nucleic acids store genetic information. Fats are a secondary energy source (more for energy storage).

Answer:

carbohydrate