Final answer:
a) microtubules. The mitotic spindle is made of microtubules, specifically kinetochore and polar microtubules, which help in aligning and separating chromosomes during cell division.
Step-by-step explanation:
The mitotic spindle is composed of microtubules, which are polymers of tubulin proteins. These microtubules form two types of spindle fibers: kinetochore microtubules and polar microtubules. Kinetochore microtubules attach to the kinetochore of the chromosomes and are responsible for aligning chromosomes at the cell center and pulling sister chromatids apart during cell division. Polar microtubules, on the other hand, extend from the centrosomes, or microtubule organizing centers (MTOCs), to the opposite poles of the cell and do not bind to kinetochores but overlap at the cell's center, contributing to the separation of cell poles.