Hello,
The answer is option A "mitosis".
Reason:
Mitosis one of the stages of a cell where the cells are separated into two chromosomes therefore putting them into a nucleus. Its not option B because cytokinesis is another part and divides one cell into two daughter cells (which means this is involved of the sex of the child). Its also not D because transcription is involved in gene expression.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit