Answer:
(a). prophase.
Step-by-step explanation:
Mitosis is a type of cell division, by which one parent cell gives rise to two identical daughter cells. Prophase represents the first stage of mitosis, during which nuclear envelope starts to disintegrate and nucleolus break down.
During late prophase, chromosomes become condensed, nucleolus and nuclear envelope disappears and mitotic spindles are formed.
Thus, the correct answer is option (a).