Final answer:
The mitotic spindle first begins to form during prophase, which is the correct answer to the student's question. This phase includes chromosome condensation, nuclear envelope breakdown, and spindle formation, pivotal steps in mitosis.
Step-by-step explanation:
The mitotic spindle begins to form during prophase of mitosis. During this phase, chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and, in animal cells, centrioles move to opposite poles of the cell. This is when the microtubules start to arrange into a spindle structure.
The spindle fibers, made of microtubules, extend from the centrioles, with some attaching to kinetochores on the centromeres of sister chromatids. Once all chromosomes are eventually lined up at the metaphase plate in metaphase, the spindle fibers help ensure that sister chromatids will separate properly, leading to the next phases of mitosis: anaphase and telophase.
The mitotic spindle begins to form in Prophase of mitosis. During prophase, the chromatin condenses into chromosomes, and the nuclear envelope breaks down. Centrioles move apart, and a spindle starts to form between them, with the spindle fibers attaching to the kinetochores of sister chromatids.
The correct option is A) Prophase.