Final answer:
The average cervical dilation per hour for nulliparas is 1 cm/hour, given that the dilation stage of labor for a vaginal birth can take 12 to 20 hours or more.
Step-by-step explanation:
The student asked about the average cervical dilation per hour for nulliparas, which are women who have not given birth before. For a vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter, which is wide enough to deliver the newborn's head. This dilation stage is the longest stage of labor and can typically take anywhere from 6-12 hours. According to the progression of labor, nulliparas on average have a slower dilation rate compared to multiparous women (those who have given birth before). Consequently, new mothers are likely to experience dilations at a rate closer to the slower end of the scale.
Given that the dilation stage may take 12 to 20 hours or more, the average dilation rate for nulliparas would be less than 1 cm per hour, which suggests that the correct answer is a. 1 cm/hour.