Which of the following is NOT a carbohydrate?

Protein is classified as a macronutrient that plays a fundamental role in the body's structure, function, and regulation of tissues and organs. It is made up of amino acids, which are fundamentally different from carbohydrates, the latter of which are primarily composed of carbon, hydrogen, and oxygen. Carbohydrates include sugars, starches, and fibers, all of which serve as energy sources or have dietary benefits.

Sugars are simple carbohydrates that provide quick energy, while starch is a complex carbohydrate that serves as a storage form of energy in plants. Fiber, though not digestible, is also a type of carbohydrate that aids in digestive health and regulates blood sugar levels. Given that protein does not fit into the carbohydrate category and serves distinct functions in the body, it is clear why it is identified in this context as not being a carbohydrate.