The correct answer is b) uterus.
The uterus is a muscular organ located in the female pelvis that plays a crucial role in fetal development. Once fertilization occurs, the fertilized egg implants itself into the lining of the uterus, where it begins to grow and develop into a fetus over the course of approximately nine months during pregnancy.
To clarify the other options: the ovaries are responsible for producing eggs and hormones; the uterine tubes (also known as fallopian tubes) are the pathways through which eggs travel from the ovaries to the uterus; the vagina is the birth canal and part of the external genitalia; and mammary glands are involved in producing milk post-delivery but are not involved in fetal development. Therefore, the uterus is indeed the site where the fetus develops during pregnancy.