How many different kinds of monomers are there in proteins?

There are 20 different type of monomers present in proteins. Amino acids are in reality the monomers of proteins. Out of the 20 amino acids, only 9 are considered essential. The other 11 are of little use to the humans. The nine of the essential amino acids required by humans are Tryptophan, lysine, Histidine, Isoleucine, Valine, Threonine, Phenylalanine, Methionine and Leucine. The amino acids combine with one another to form different types of proteins. These proteins satisfy a number of our physical functional needs.

The correct answer is c. orbital shape. "the angular momentum quantum number l describes the shape of an orbital. it corresponds to each orbital type, i.e. (0,1,2,−1)=(s,p,d,f,g,"