Which does the letter a represent in the diagram?

The indicate part in the figure is spindle fiber. Hence, the correct answer is spindle.

The spindle fibers are the thread like structures, which attach to the chromosomes at the kinetochore. The attachment of the spindle fibers takes place at the time of the metaphase during the division of the cells. It helps in the pulling of the chromatids to the opposite poles of the cell during the anaphase of the cell division.

this would be called the responsiveness