What elements are present in proteins?

Proteins are composed of specific building blocks known as amino acids, which contain the elements carbon, hydrogen, oxygen, and nitrogen. These four elements are integral to the structure of amino acids and, consequently, proteins formed by linking amino acids together through peptide bonds. In addition to these four elements, some amino acids may contain sulfur; however, sulfur is not a universal element found in every protein. Thus, the presence of carbon, hydrogen, oxygen, and nitrogen is fundamental and consistent across all proteins, making the choice listing these elements correct.

Other groups of elements mentioned in the other options, such as sodium, potassium, calcium, magnesium, iron, sulfur, and chlorine, are important for various cellular functions and processes but are not primary components of proteins themselves.