1)8 nucleated and 7 celled structure is called female gametophyte or embryosac of angiosperm .
2) process of development of embryosac is called as megagametogenesis
3) so, correct answer is B.

Embryosac is also known as Megagametophyte.

Embryosac:
- The embryosac is a structure found in the ovule of flowering plants. It is a female gametophyte that contains the egg cell or ovum, which is fertilized by the sperm to form the zygote.
- The embryosac is a crucial part of the reproductive process in angiosperms and plays a vital role in sexual reproduction.

Megagametophyte:
- The megagametophyte is the female gametophyte or the embryo sac in flowering plants.
- It is a multicellular structure that develops from the megaspore, which is produced by the megasporangium.
- The megagametophyte is usually located within the ovule and contains the egg cell, synergids, antipodal cells, and central cell.
- The egg cell is the female gamete that fuses with the male gamete (sperm) during fertilization to form the zygote, which develops into an embryo.
- The synergids help guide the pollen tube to the egg cell, and the antipodal cells have a role in nourishing the developing embryo.
- The central cell contains two polar nuclei which fuse with one of the sperm nuclei during fertilization to form the endosperm, a tissue that provides nourishment to the developing embryo.

Importance of Embryosac:
- The embryosac is essential for sexual reproduction in flowering plants.
- It provides a protected environment for the development of the female gametes and facilitates the fusion of the egg cell with the sperm.
- The successful fertilization of the egg cell leads to the formation of the zygote, which develops into an embryo and eventually a seed.
- The endosperm, formed by the fusion of the polar nuclei with the sperm, provides nourishment to the developing embryo.
- The embryosac also plays a role in determining the genetic composition of the offspring, as the egg cell and sperm contribute half of the genetic material each.

In conclusion, the embryosac is also known as the megagametophyte in flowering plants. It is a female gametophyte structure that contains the egg cell and other supporting cells necessary for fertilization and seed development.