Final answer:
Microtubules form the mitotic spindle during cell division, and they originate from the centrosome.
Step-by-step explanation:
The cytoskeletal structure that makes up the mitotic spindle is the microtubules. Microtubules are the thickest of the three main types of cytoskeletal fibers and are most commonly made of alpha and beta-tubulin dimers that form hollow cylinders. These structures are crucial for maintaining the cell's shape, allowing organelle movement, and they are especially important during cell division as they form the mitotic spindle, which is essential for properly segregating chromosomes to daughter cells. The mitotic spindle is formed from microtubules that originate from an area near the nucleus called the centrosome. Hence, the correct answer is C. microtubules.