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

Explanation

**Step-by-Step Solution:**

1. **Understanding Protein Structure**: Proteins have four levels of structure: primary, secondary, tertiary, and quaternary.
- **Primary Structure**: This is the linear sequence of amino acids in a polypeptide chain.
- **Secondary Structure**: This involves the folding of the polypeptide chain into structures like alpha helices and beta sheets, stabilized by hydrogen bonds.
- **Tertiary Structure**: This is the overall three-dimensional shape of a single polypeptide chain, formed by further folding and interactions between secondary structures.
- **Quaternary Structure**: This involves the assembly of multiple polypeptide chains into a functional protein complex.

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