The correct option is D Embryo
Ovum and sperm are female and male reproductive cell respectively. After fertilization (fusion of male and female gamete), a zygote is formed. Once the zygote is formed, the endometrium of the uterus increase in thickness and it is considered that the uterus is now ready for embryo implantation. Increase in thickness of endometrium means more vascular endometrium, causing more blood supply.
Therefore, the correct answer is option B.