**Core Concept**
Vaginal delivery is impossible in cases where the fetal head or body is not capable of passing through the birth canal due to anatomical or obstetric complications. This can be due to various factors such as fetal macrocephaly, multiple gestations, or cephalopelvic disproportion.

**Why the Correct Answer is Right**
The correct answer is a condition where the fetal head is too large to pass through the birth canal. This can occur in cases of fetal macrocephaly, where the fetal head is significantly larger than the average size, or in cases of cephalopelvic disproportion, where the pelvic inlet is too small to accommodate the fetal head. This condition requires a cesarean section to ensure a safe delivery for both the mother and the baby.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as fetal macrosomia (a condition where the baby is significantly larger than average) can still be delivered vaginally in many cases, especially if the mother is able to push effectively.
**Option B:** This option is incorrect as multiple gestations (twins, triplets, etc.) can often be delivered vaginally, especially if the babies are smaller and the mother is able to push effectively.
**Option C:** This option is incorrect as a history of previous cesarean section is not an absolute contraindication to vaginal delivery, and in some cases, a trial of labor after cesarean (TOLAC) may be attempted.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that vaginal delivery is not always possible, and a thorough assessment of the mother's pelvis and the baby's size and position is essential to determine the safest delivery method.

**Correct Answer:** C. Cephalopelvic disproportion.