The correct option is C Megaspore by mitosis
Female gametes in the angiosperms are formed by the mitotic division of the megaspore.
The megaspore mother cell (MMC) doesn’t directly give rise to female gametes. Instead, megaspore mother cell first undergoes meiotic division and gives rise to the megaspore which then undergoes mitotic divisions to give rise to the female gamete.
Nucellus contains somatic cells are the non-reproductive cells of an organism. So female gametes are not formed by the somatic cells either.