From which structure does a single embryo sac in an ovule generally form?

Megaspore

d) Megaspore

The single embryo sac in an ovule generally forms from a megaspore. A megaspore is a specialized type of spore produced within the ovule. During the development of the ovule, one of the cells undergoes meiosis to produce a megaspore. This megaspore then undergoes further development and gives rise to the embryo sac or female gametophyte.

The megaspore undergoes several rounds of mitotic divisions to form the embryo sac, which is a multicellular structure within the ovule. The embryo sac contains the female reproductive cells, including the egg cell (ovum) and synergids, which are involved in the fertilization process.

It's important to note that in most flowering plants, including angiosperms, the embryo sac is a haploid structure, meaning it has half the number of chromosomes compared to the somatic cells of the plant. This haploid nature is crucial for sexual reproduction as it allows for the fusion of gametes during fertilization, resulting in the formation of a diploid zygote.

Therefore, the correct answer is: d) Megaspore.