The correct option is A Tubulin
Spindle fibres of mitotic cells are made up of microtubules. It consists of protein tubulin that maintains cell's shape, serves as tracks for organelle movement & helps in cell division by getting attached to the centromeres of bivalents. They are arranged on equator due to congression movements. The movement of bivalents or chromosomes occurs towards the poles as a result of spindle fibres contraction.