What happens during anaphase?

During anaphase, the key event is the separation of sister chromatids, which are the identical copies of a chromosome formed during DNA replication. These chromatids are pulled apart by the spindle fibers and migrate towards opposite poles of the cell. This movement ensures that each new daughter cell will receive an identical set of chromosomes during cell division.

The other events described in the other choices occur at different stages of the cell cycle. Chromosome replication takes place during the S phase of interphase, prior to mitosis. The reformation of the nuclear envelope occurs during telophase, after the chromatids have separated. Additionally, the alignment of chromosomes at the cell's equator occurs during metaphase, which precedes anaphase. Thus, the correct answer accurately reflects the specific events of anaphase in the cell division process.