During which cell cycle phase does the cell's nucleus replicate its chromosomes?

The correct answer is interphase because this is the phase of the cell cycle where the cell prepares for division by replicating its DNA. More specifically, during the S (synthesis) phase of interphase, the chromosomes are duplicated, resulting in two identical sets that are ready for distribution to daughter cells during mitosis.

Prophase, while an important stage in mitosis, is when the replicated chromosomes condense and become visible, but actual replication has already occurred during interphase. Metaphase and telophase are specific stages in mitosis: metaphase is characterized by the alignment of chromosomes at the cell's equatorial plane, while telophase involves the reformation of the nuclear envelope around the two sets of chromosomes that have been separated. Hence, the replication of chromosomes happens exclusively during interphase.