Answer: The correct answer for the blank is-
Prophase.
M phase (mitotic phase) is the second phase of cell cycle (the first being interphase, which contains G1, S, and G2 stages).
In the M phase, the parent cell is divided into two daughter cell (through a sequence of 5 stages in the order- Prophase, metaphase, anaphase, telophase, and cytokinesis). The daughter cells have same number of chromosomes as that of the parent cell.
Prophase marks the beginning of M phase. In this phase, nuclear envelope is broken down and chromatin becomes condensed into visible chromosomes. Mitotic spindle formation is also initiated in this phase.
Thus, prophase is the right answer.