## Core Concept
The question tests understanding of protein structure, specifically the types of secondary structures that proteins can form. Alpha helix and beta pleated sheet are fundamental concepts in biochemistry, representing two of the most common types of secondary structures in proteins.

## Why the Correct Answer is Right
Alpha helix and beta pleated sheet are indeed examples of **secondary structures** of proteins. The secondary structure of a protein refers to the local arrangements of a protein's polypeptide chain, primarily through hydrogen bonding. The alpha helix is a spiral structure stabilized by hydrogen bonds between amino acids, while the beta pleated sheet is a planar structure with hydrogen bonds between adjacent strands. Both are crucial for understanding the overall 3D conformation and function of proteins.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and does not provide any information to evaluate.
- **Option B:** This option is also blank and cannot be assessed.
- **Option C:** Similarly, this option does not offer any relevant information.
- **Option D:** This option is blank as well.

## Clinical Pearl / High-Yield Fact
Understanding the secondary structures of proteins, such as alpha helices and beta pleated sheets, is essential for predicting protein function and understanding diseases caused by protein misfolding, such as Alzheimer's disease and prion diseases. A key point to remember is that these structures are stabilized by **hydrogen bonds**.

## Correct Answer Line
**Correct Answer: C. Secondary structure**.