Assertion (A): Only the functional megaspore undergoes three mitotic divisions to form a mature embryo sac.

Reason (R): Three of the four megaspores produced during meiosis degenerate, leaving one megaspore to develop into the embryo sac.

Both A and R are true, and R is the correct explanation of A.

The correct answer is Option (1) - Both A and R are true, and R is the correct explanation of A.

The embryo sac, also known as the female gametophyte, is a crucial structure within the ovule of a flowering plant. It is formed through the process of megasporogenesis, which involves the differentiation and development of a single megaspore mother cell within the ovule. The megaspore mother cell undergoes meiosis to produce four haploid megaspores. Out of these four megaspores, three degenerate and only one megaspore survives and develops into the embryo sac.

The surviving megaspore undergoes three rounds of mitotic divisions without cytokinesis, resulting in the formation of eight haploid nuclei. These nuclei become organized into different cells within the embryo sac through the formation of cell walls. The organization of the nuclei and cells within the embryo sac varies among different species, but a common pattern consists of three antipodal cells at the chalazal end, two synergid cells and egg cell near the micropyle, a centrally located large central cell containing two polar nuclei.

The embryo sac plays a vital role in sexual reproduction in angiosperms. It contains the female gametes, including the egg cell, which fuses with the sperm cell during fertilization to form the zygote. The central cell, with its two polar nuclei, is involved in the process of double fertilization, where it fuses with one of the two sperm cells to form the endosperm, which nourishes the developing embryo.

In summary, the embryo sac is the female gametophyte in flowering plants, housing the female gametes and playing a crucial role in the process of sexual reproduction.