Which of the following is NOT a macromolecule?

Macromolecules are large, complex molecules that are vital for biological processes, and they include carbohydrates, lipids, and proteins. These macromolecules are made up of numerous smaller units or subunits, such as sugars in carbohydrates, fatty acids in lipids, and amino acids in proteins. They play essential roles in structure, function, and regulation of the body's tissues and organs.

Water, on the other hand, is a simple molecule consisting of two hydrogen atoms bonded to one oxygen atom (H2O). While it is crucial for life and serves as a solvent and medium for various biochemical reactions, it does not meet the criteria of a macromolecule due to its relatively small size and lack of complex structure. Therefore, water is not classified as a macromolecule, making it the correct answer to identify as NOT being a macromolecule.