**Core Concept**
The structural proteins, such as **collagen** and **keratin**, play a crucial role in maintaining the shape of a cell or in the formation of matrices in the body. These proteins have unique properties that allow them to provide support and structure to various tissues. The shape of these proteins is essential for their function.

**Why the Correct Answer is Right**
The correct answer is related to the **fibrous structure** of these proteins, which allows them to provide support and strength to tissues. **Collagen**, for example, has a triple-helix structure that gives it great tensile strength, while **keratin** has a coiled-coil structure that provides rigidity. These structures enable the proteins to maintain the shape of cells and form matrices.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the shape of structural proteins.
**Option B:** This option is also incorrect as it does not relate to the shape of structural proteins.
**Option D:** This option is incorrect because it is not a characteristic of structural proteins.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **fibrous proteins** have a specific shape that allows them to provide support and structure to tissues. This is essential for maintaining the integrity of various tissues in the body.

**Correct Answer:** D. Fibrous.