The correct option is A Glycine
Proteins are polymers of amino acids. Animals cannot synthesize seven amino acids. They are called essential amino acids (lysine, methionine, leucine, isoleucine, valine, tryptophan and phenylalanine). In humans, threonine is also an essential amino acid. Arginine and histidine are semi indispensable because they are slow to be formed in humans. Rest of the amino acids which can be synthesized in the animal cells are called as non essential amino acids.