Final answer:
The posterior fornix of the vagina is closest to the pouch of Douglas, as it is adjacent to the rectouterine pouch.
Step-by-step explanation:
The pouch of Douglas, also known as the rectouterine pouch, is a space in the female pelvis. When considering which part of the vagina is closest to the pouch of Douglas, it is important to understand the anatomy of the vagina, particularly the sections known as the anterior and posterior fornices. The fornix is the superior portion of the vagina that surrounds the cervix, forming a border with the lower uterine segment. The posterior fornix is adjacent to the rectouterine pouch, making it the closest vaginal part to the pouch of Douglas. Therefore, the correct answer to the question is B) Posterior fornix.