**Core Concept**
Chemotaxis is the movement of cells, particularly immune cells, towards a chemical stimulus. In the context of immune response, chemotaxis is crucial for the recruitment of leukocytes to sites of inflammation. Chemoattractants are chemical signals that induce chemotaxis by binding to specific receptors on the surface of immune cells.

**Why the Correct Answer is Right**
The correct answer is a chemokine, which is a family of small cytokines that induce chemotaxis in immune cells. Chemokines bind to specific G-protein coupled receptors (GPCRs) on the surface of leukocytes, triggering a signaling cascade that leads to the activation of downstream effectors, including the polymerization of actin filaments and the formation of pseudopodia, ultimately resulting in cell movement towards the chemoattractant source. This process is essential for the recruitment of leukocytes to sites of inflammation and infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because, while it may be a cytokine, it is not typically classified as a chemoattractant.
* **Option B:** This is incorrect because it is a type of growth factor, not a chemoattractant.
* **Option C:** This is incorrect because it is a type of adhesion molecule, not a chemoattractant.

**Clinical Pearl / High-Yield Fact**
Chemokines play a crucial role in the recruitment of leukocytes to sites of inflammation, and their dysregulation has been implicated in various inflammatory and autoimmune diseases, including asthma, atherosclerosis, and multiple sclerosis.

**Correct Answer:** C. Chemokine. Chemokine.