Ans.
The amino acids are organic molecules, having two functional groups, a carboxyl group (-COOH) and an amino group (-NH2) along with a side chain (R group) and a hydrogen atom. The amino acids are known as building blocks of protein molecules as join together to make protein.
Two amino acids join together with the help of a peptide bond, formed between carboxyl group of one amino acid and amino group of another amino acid.
Thus, the correct answer is option B). 'carboxyl and amino group.'