What is formed from a zygote 4-5 days after fertilization?

The correct response is a blastocyst, which is formed from a zygote approximately 4-5 days after fertilization. After the initial fertilization process, the zygote undergoes several rounds of cell division, creating a structure known as a morula. As this structure continues to develop, it forms a hollow sphere of cells called a blastocyst. This blastocyst is a critical stage in embryo formation, as it is the structure that will later implant into the uterine wall, leading to further developmental processes.

In contrast, an ovum refers to the female reproductive cell and does not describe a stage formed post-fertilization. The embryo is the stage that follows the blastocyst, generally beginning once implantation occurs, while the fetus is a later stage of development, typically starting around eight weeks post-fertilization. Thus, the formation of a blastocyst is a key part of the early developmental timeline shortly after fertilization, making it the correct answer.