**Core Concept**
The fetal skull's diameter is crucial for understanding obstetric anatomy, particularly during labor and delivery. The mento-vertical diameter (MVD) is the longest diameter of the fetal skull, which plays a significant role in determining the passage of the fetal head through the birth canal.

**Why the Correct Answer is Right**
The mento-vertical diameter is the longest diameter of the fetal skull, measuring approximately 13-14 cm. It is the distance from the chin (mentum) to the highest point of the vertex of the skull (vertex). This diameter is essential because it determines the size of the fetal head that can pass through the maternal pelvis during delivery. The MVD is critical in assessing fetal head size and predicting potential issues during labor.

**Why Each Wrong Option is Incorrect**
**Option A:** Occipito frontal is not the longest diameter of the fetal skull. It is one of the other diameters, but not the longest.
**Option B:** Submentobregmatic is the diameter from the chin to the bregma (the point where the frontal and two parietal bones meet), which is not the longest diameter of the fetal skull.
**Option C:** Suboccipito frontal is not a standard measurement of fetal skull diameter and is not relevant in this context.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the longest diameter of the fetal skull is the mento-vertical diameter, which is approximately 13-14 cm. This knowledge is crucial in obstetrics, particularly during labor and delivery, when assessing fetal head size and predicting potential complications.

**✓ Correct Answer: D. Mento vertical.