## **Core Concept**
The primary structure of a protein refers to the sequence of amino acids linked by peptide bonds. This linear sequence is fundamental to the protein's overall 3D structure and function. The primary structure is determined by the covalent bonds between amino acids.

## **Why the Correct Answer is Right**
The correct answer involves **peptide bonds**, which are covalent bonds formed between the carboxyl group of one amino acid and the amino group of another. This type of bond is crucial for linking amino acids in a specific sequence, thereby defining the primary structure of a protein. Peptide bonds are formed through a dehydration synthesis reaction.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Ionic bonds are involved in the tertiary structure of proteins, where they help stabilize the 3D conformation. They are not primarily responsible for the primary structure.
- **Option B:** Hydrogen bonds play a significant role in the secondary and tertiary structures of proteins, contributing to the stability of alpha helices and beta sheets, but not the primary structure.
- **Option C:** Disulfide bridges are also relevant to the tertiary structure, providing covalent links between cysteine residues in different parts of the protein chain, which helps stabilize the 3D structure.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the primary structure is defined by peptide bonds, changes in this sequence, such as those caused by mutations, can significantly affect protein function and are associated with various diseases.

## **Correct Answer:** . Peptide bonds.