During development, an embryo is embedded in the

a. ovary.

b. cervix.

c. vagina.

d. endometrium.

The best answer is d. endometrium.

Endometrium is the innermost layer in the Uterus, and it is found in the whole uterus.

For comparison, Cervix is the structure on the "bottom" of the uterus, vagina is below it (outside of the womb) and the ovaries if where the unfertilized eggs are.