Final answer:
The correct answer is d) Centrosomes, spindle. This occurs during the prophase of mitosis, where the nuclear envelope breaks down, and the centrosomes begin to move to opposite poles with the formation of the mitotic spindle.
Step-by-step explanation:
During prophase of mitosis, the nuclear membrane breaks down into small vesicles, and the nucleolus becomes less visible. Furthermore, the two centrosomes move apart from each other, and the mitotic spindle begins to form. Therefore, the correct answer is d) Centrosomes, spindle.
Prophase marks several crucial events: the nuclear envelope breaking down, the nucleolus fading, centrosomes migrating to the cell's opposite poles, and the assembly of the mitotic spindle made of microtubules which further separate the centrosomes. Additionally, sister chromatids begin to coil more tightly, making them visible under a light microscope.