The correct option is A Centrosome
Though chromosome forms an integral part of the nucleus, it is not an organelle.
Centrosome is an organelle that serves as the microtubule organising center (MTOC) of the cell and regulates the progression of cell cycle. During mitosis, as the nuclear membrane disintegrates, the centrosome nucleated microtubules interact with the chromosomes for building up of the mitotic spindle.