The correct option is C Both a and b
Spindle fibers are highly active during mitosis. They migrate throughout the cell and direct chromosomes to go where they need to go. In anaphase, spindle fibers shorten and pull sister chromatids towards the spindle poles. Separated sister chromatids move towards opposite cell poles. Chromosomal fibers pull chromosomes towards the pole while interzonal fibers push the chromosomes towards the poles. Hence, both are required during anaphase to move chromosomes from equator to the poles.