**Core Concept**
Flagella are whip-like structures found on certain microorganisms, such as bacteria and some eukaryotic cells. They play a crucial role in the motility of these cells, enabling them to move towards or away from their environment. The movement of flagella is achieved through a rotational motion, which is generated by the interaction of the flagellum with the surrounding medium.

**Why the Correct Answer is Right**
The primary function of flagella is to facilitate locomotion, allowing cells to move through their environment. This is achieved through the rotation of the flagellum, which creates a force that propels the cell forward. The attachment of flagella to the cell membrane is crucial for this function, but it is not the primary role of flagella. In fact, flagella are often involved in the process of chemotaxis, where cells move towards or away from chemical stimuli.

**Why Each Wrong Option is Incorrect**
**Option A:** Locomotion is indeed a true function of flagella, as they enable cells to move through their environment. This is the primary role of flagella, making option A incorrect.
**Option C:** Flagella are indeed protein in nature, composed of a protein called flagellin that makes up the flagellum. This is a true statement, making option C incorrect.
**Option D:** Flagella can be antigenic, meaning they can stimulate an immune response. However, this is not the primary function of flagella, making option D incorrect.

**Clinical Pearl / High-Yield Fact**
Flagella are a key feature of certain bacteria, such as Pseudomonas aeruginosa and Vibrio cholerae. These bacteria use their flagella to move through their environment, which is essential for their survival and pathogenicity.

**Correct Answer:**
✓ Correct Answer: B. Attachment