Proteins perform a diverse of functions and the function of a protein is determined by its tertiary structure.
Proteins are the main structural components of muscles. They also serve as significant contributions to energy resource, protein catabolism mainly occurs during fasting or excessive muscular activity.
Other functions of proteins include, they work as enzymes (Eg: pepsin), hormones (Eg: insulin) and receptors (Eg: membrane receptors.).
Thus, the options (a), (d) and (b) are incorrect.
Proteins are the main structural components of muscles. They also serve as significant contributions to energy resource, protein catabolism mainly occurs during fasting or excessive muscular activity.
Therefore, the correct option is – C. proteins.