Final answer:
The correct answer is b.The cervical-vaginal junction is referred to as the external os, which is the opening of the cervical canal into the vagina.
Step-by-step explanation:
The cervical-vaginal junction refers to the area where the cervix of the uterus meets the vagina. The cervix protrudes into the vagina and has a small canal that runs the length of the cervix, connecting the uterine cavity with the lumen of the vagina. This channel is known as the cervical canal and the opening into the vagina is referred to as the external os, which can be identified as option b) Os.
During the reproductive process, this section of the cervix plays a crucial role; it allows semen to enter the uterus from the vagina, and during childbirth, it becomes dilated to allow the passage of a baby from the uterus into the vagina.