During denaturation of proteins, only............... structure of protein is retained.

Explanation

To answer the question regarding which structure of proteins is retained during denaturation, we can follow these steps:

### Step 1: Understand Protein Structure
Proteins have four levels of structure:
1. **Primary Structure**: The sequence of amino acids linked by peptide bonds.
2. **Secondary Structure**: Local folding of the polypeptide chain into structures such as alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
3. **Tertiary Structure**: The overall three-dimensional shape of a polypeptide, formed by interactions between R groups (side chains) of amino acids, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.
4. **Quaternary Structure**: The arrangement of multiple polypeptide chains into a functional protein.
...