Which of the following statements about primary, secondary, and tertiary protein structures is incorrect? The tertiary structure of a protein is stabilized by interactions between R groups of different amino acids. That's true. Remember, it's close and distant R groups that are helping with the stabilization.
Now here, the folding of a peptide into the tertiary structure destroys its secondary structure. Remember this is incorrect. The tertiary structure being formed does not erase the primary and secondary structures. It's just an evolution, it's becoming more complex in terms of the polypeptide chain. Okay? It's not undoing the primary and secondary structures.
Due to the folding of the peptide chain, distant R groups come close and interact with each other. That is true. With this folding in and moving and twisting, some R groups are going to have greater interactions than before.
A folded peptide or protein can have alpha helices and beta pleated sheets at the same time. Yes. These polypeptide chains can be many amino acids long, portions of it can contain alpha helices, and portions of it can contain beta pleated sheets at the same time. So this is indeed possible. So, this is correct. The only statement here that is incorrect is option B.