## **Core Concept**
The question tests understanding of protein structure and the effects of heat on these structures. Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. Denaturation of proteins involves the disruption of their tertiary and quaternary structures, which can be caused by various factors including heat.

## **Why the Correct Answer is Right**
The correct answer, **D. Primary structure**, refers to the sequence of amino acids in a protein, which is determined by peptide bonds. These peptide bonds are strong covalent bonds that are not broken by heat up to 100 degrees Celsius. Therefore, the primary structure of a protein remains intact even after heating to 100 degrees Celsius.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Secondary structure refers to local arrangements of a protein's polypeptide chain, such as alpha helices and beta sheets, which are stabilized by hydrogen bonds. These can be disrupted by heat, leading to denaturation.
- **Option B:** Tertiary structure is the overall 3D shape of a single protein molecule, also stabilized by various interactions including hydrogen bonds, ionic bonds, and disulfide bridges. Heat can disrupt these interactions, causing denaturation.
- **Option C:** Quaternary structure refers to how polypeptide chains are arranged in a multi-subunit protein. This structure is also stabilized by interactions similar to those in the tertiary structure and can be disrupted by heat.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while the primary structure of a protein is stable to heat, the function of a protein is closely related to its tertiary and quaternary structures. Denaturation can lead to loss of function, which is crucial in understanding disease processes and protein-related therapies.

## **Correct Answer:** D. Primary structure