Final answer:
The covalent linkage between two amino acids in a protein is called a peptide bond, and it is formed by a dehydration synthesis reaction.
Step-by-step explanation:
The covalent linkage between two adjacent amino acids in a protein is called a peptide bond. This type of bond is a covalent bond that forms through a dehydration synthesis reaction, where the carboxyl group of one amino acid reacts with the amino group of another, leading to the release of a water molecule. Therefore, the correct answer to the question is D. peptide bond.
A peptide bond specifically links the carboxyl end of one amino acid with the amino end of another, highlighting its crucial role in forming proteins. As more amino acids link together via peptide bonds, a protein's unique structure begins to form, which is essential for its function. The structures that are shorter than about 100 amino acids are often referred to as polypeptides rather than proteins.