Ans. (A). Myofibrillar proteins in muscle.
Proteins are polymer of amino acids. Each amino acid is composed of an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a R-group of different nature. As each amino acid has a amino group, nitrogen is a major constituent of all proteins.
On the other hand, carbohydrates (such as cellulose and glucose) and fats are composed of hydrogen, oxygen, and carbon atoms.
Hence, limited nitrogen will most affect the myofibrillar proteins present in the muscles.
Thus, the correct answer is option (A).