A weight lifter needs to build muscle mass. which macromolecule would be the best source of building materials for the weight lifter's body?

The answer is protein

There are 3 macromolecules that the body needs: carbohydrate, protein, and fat. Protein is used to make new cells. The weightlifter needs to build muscle mass, and muscle fiber is made mostly of protein. In fact, the main source of protein in animals is their meat/muscle.