Which chemical compound serves multiple functions, such as building muscles and transporting nutrients in the body?

The correct answer is protein, which serves multiple essential functions in the body, including building muscles and transporting nutrients. Proteins are made up of long chains of amino acids, and they play a crucial role in muscle growth and repair. As a major structural component of muscles, proteins are vital for the development of muscle fibers and overall strength.

In addition to their role in muscle formation, proteins are involved in transporting a wide range of substances across cell membranes and within the bloodstream. For example, hemoglobin, a type of protein in red blood cells, is responsible for transporting oxygen from the lungs to the rest of the body. Other proteins can transport nutrients like iron and fatty acids, facilitating vital metabolic processes.

While amino acids are essential components of proteins and indeed serve multiple functions as building blocks for protein synthesis, they do not encompass the entirety of the protein's role in the body. Lipids and carbohydrates, while important macronutrients, serve different primary functions; lipids mainly provide energy storage and insulation, while carbohydrates are primarily responsible for supplying quick energy. Thus, proteins stand out as the key compound for both muscle building and transporting nutrients effectively within the body.