Final answer:
The developmental stage that follows after 4-5 days when the zygote becomes an embryo is blastocyst formation. The zygote undergoes cleavage to form a morula and then a blastocyst, which is ready for implantation into the uterine wall for further development.
Step-by-step explanation:
After 4-5 days when the zygote becomes an embryo, the next developmental stage is blastocyst formation. The embryonic development process begins with fertilization, during which a sperm fuses with an egg to produce a zygote. This zygote undergoes cleavage, dividing into more cells without increasing the overall size of the organism. The successive cell divisions lead to the formation of a ball of cells known as the morula.
As the zygote travels down the fallopian tube toward the uterus, it continues to divide and eventually forms the blastocyst. The blastocyst is characterized by a fluid-filled cavity and differentiated cells. This stage is crucial for the implantation in the uterine wall, which allows the embryo to start obtaining nutrients necessary for further growth and development.
Therefore, the correct answer to the question is a. Blastocyst formation.