Final answer:
During mitosis, the nuclear envelope breaks down to allow the chromosomes to be free in the cytoplasm.
Step-by-step explanation:
The correct answer is Dissolving of the nuclear envelope. During mitosis, the nuclear envelope breaks down, allowing the chromosomes to be free in the cytoplasm. This is necessary for the chromosomes to align and separate properly during cell division. The other options listed - separation of chromatids, division of the cytoplasm, and formation of chromosomes - all occur during mitosis.
Learn more about Mitosis in Biology