**Core Concept**
The mitotic spindle is a complex structure composed of microtubules that plays a crucial role in the separation of chromosomes during cell division. The formation of the mitotic spindle is essential for the accurate segregation of genetic material.

**Why the Correct Answer is Right**
The correct answer is **Tubulin**. Tubulin is the main protein component of microtubules, which are dynamic polymers that make up the mitotic spindle. During mitosis, tubulin subunits polymerize to form microtubules that interact with kinetochores on sister chromatids, ultimately leading to the separation of chromosomes. The microtubules of the mitotic spindle are stabilized by motor proteins, such as dynein and kinesin, which facilitate the movement of chromosomes to opposite poles of the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** **Centriole**. While centrioles are involved in the formation of the mitotic spindle, they are not the primary protein component. Centrioles act as nucleation sites for microtubule polymerization, but they do not directly participate in spindle formation.

**Option B:** **Actin**. Actin filaments are involved in cell motility and cytoskeletal dynamics, but they are not directly involved in the formation of the mitotic spindle.

**Option C:** **Kinesin**. Kinesin is a motor protein that interacts with microtubules and plays a role in the movement of chromosomes during mitosis. However, it is not the primary protein component of the mitotic spindle.

**Clinical Pearl / High-Yield Fact**
The mitotic spindle is a highly dynamic structure that is regulated by a complex interplay of proteins, including tubulin, motor proteins, and regulatory proteins such as Aurora kinases. Understanding the molecular mechanisms underlying mitotic spindle formation is essential for the accurate diagnosis and treatment of disorders related to cell division.

**Correct Answer: D. Tubulin**