The correct answer is B.
Formation of an embryo begins when the sperm cell meets an egg. When they meet, a zygote is first formed. A zygote is a single diploid cell created when two haploid cells merge.
The zygote begins to divide , and when this division starts, the zygote turns into an embryo. So an embryo is an organism that is at the early stages of development and which is not able to survive on its own.
The term embryo is used only to refer to eukaryotes or organisms that are multi cellular.