**Core Concept**
The cervical opening is a critical anatomical landmark that varies among individuals. In nulliparous women, the cervix is typically a smaller, more firm structure compared to multiparous women. The cervical opening in nulliparous women is usually smaller and more constricted.

**Why the Correct Answer is Right**
The correct answer is based on the anatomical and physiological differences between nulliparous and multiparous women. Nulliparous women have not given birth before, and their cervix is not as effaced or dilated as it would be in a multiparous woman. The smaller cervical opening in nulliparous women is due to the presence of a thicker and more fibrous cervix. This thicker cervix is less prone to dilation and effacement, which are necessary for labor and delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the cervical opening in nulliparous women. The cervical opening in nulliparous women is not typically larger than 2.5 cm.
**Option B:** This option is incorrect because it is not a specific measurement or description of the cervical opening in nulliparous women. A more precise answer would provide a clear measurement or description of the cervical opening.
**Option C:** This option is incorrect because it is not a standard measurement or description of the cervical opening in nulliparous women. The correct answer should provide a clear and specific measurement or description of the cervical opening.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the cervical opening in nulliparous women is typically smaller and more constricted than in multiparous women. This anatomical difference is critical in obstetrics, as it affects the likelihood of successful labor and delivery.

**Correct Answer: D. 2.5 cm**