**Core Concept**
The cervical canal's diameter is a critical anatomical parameter in obstetrics, particularly during labor and delivery. The cervix undergoes significant changes in shape and size to facilitate fetal passage through the birth canal.

**Why the Correct Answer is Right**
During complete cervical dilation, the cervix is fully effaced and dilated to approximately 10 cm in diameter. This allows for the passage of the fetal head through the birth canal. The cervix's transformation is due to the breakdown of collagen fibers and the remodeling of the cervical stroma, facilitated by the production of matrix metalloproteinases (MMPs) and the activity of cervical stroma cells.

**Why Each Wrong Option is Incorrect**
**Option A:** 2 cm is too small for complete cervical dilation, which is a critical step in labor and delivery.
**Option B:** 5 cm is partially dilated but not completely dilated, which is not sufficient for fetal head passage.
**Option C:** 12 cm is excessively large and not a typical measurement for the cervical canal diameter.

**Clinical Pearl / High-Yield Fact**
Remember the "10-10-10" rule: a completely dilated cervix is approximately 10 cm in diameter, and the fetal head should be visible at the introitus (10 cm from the pubic symphysis), with the woman in a semi-recumbent position and the bed in a 10-degree tilt.

**Correct Answer: D. 10 cm**