Final answer:
The centrosome, containing a pair of centrioles, acts as the principal microtubule-organizing center in chromosomes and is responsible for forming the mitotic spindle during cell division.
The correct option is A) Centrosome.
Step-by-step explanation:
The principal microtubule-organizing center in chromosomes during cell division is the centrosome. The centrosome contains a pair of centrioles, with each centriole being a cylinder composed of nine triplets of microtubules. These centrioles are arranged perpendicular to each other.
During mitosis, the centrosomes serve as the main centers from which the mitotic spindle fibers emanate, with some connecting to the kinetochore on the chromosomes through kinetochore microtubules.
The correct option of this discussion is A) Centrosome.