Final answer:
The key nutrient found in carbohydrates, proteins, and nucleic acids that forms biomolecules is carbon. Carbon makes up about 50% of the cell's composition and is integral to the structure of all macromolecules.
Step-by-step explanation:
The nutrient that is part of carbohydrates, proteins, and nucleic acids, and that forms biomolecules, is carbon. Carbon is essential to life as it accounts for about 50% of the composition of a cell. It has the unique ability to form four bonds with itself and with other elements which allows for the formation of a vast array of molecules necessary for life. These include macromolecules such as carbohydrates, proteins, nucleic acids, and lipids. Although nitrogen is also a vital element that makes up approximately 12% of the cell's dry weight and is found in proteins and nucleic acids, it is carbon that is the major element in all these organic compounds.