Final answer:
The union of sperm and ovum results in the formation of a zygote, which is the earliest stage of prenatal development. This single-cell will undergo several divisions to become an embryo and further develop into a fetus and eventually an infant.
Step-by-step explanation:
Fertilization (fusing of one sperm and an ovum) results in a zygote. The correct answer to the student's question is D. zygote.
The zygote is a single-cell structure that is created when a sperm and an egg merge at conception. This single-cell will undergo numerous cell divisions, eventually leading to an embryo which further develops into a fetus. This process is the very beginning of prenatal development. The correct order of prenatal development is zygote, embryo, and then fetus. During the approximately nine-month period of pregnancy, the zygote develops into an embryo, which then forms into a fetus, culminating in the birth of an infant.
Implantation occurs after the zygote stage, when the then-developed embryo attaches itself to the lining of the uterus. It's important to differentiate these stages to understand the process of human development comprehensively. Each stage is characterized by significant growth and developmental changes, governed by genetic material inherited from the gametes and influenced by environmental factors.