Megasporangium is equivalent to an ovule.

The ovule is the structure in seed plants that contains the female gametophyte and the megasporangium, which is the structure that produces megaspores. Megaspores are the precursor cells that develop into female gametophytes, which produce the egg cells that are fertilized by sperm cells to form the zygote and ultimately the embryo.

The nucellus is the central part of the ovule that surrounds the megasporangium and the female gametophyte. The embryo sac is the female gametophyte itself, and the fruit is the mature ovary of a flowering plant that contains seeds.

So, option B, "ovule," is the correct answer.