**Core Concept:** Placenta morphology plays an essential role in identifying the type of twins in a multiple gestation pregnancy. In twin pregnancies, the placenta types can provide clues to the relationship between the fetuses.

**Why the Correct Answer is Right:** Identical twins (monozygotic twins) arise from a single fertilized egg that divides into two embryos, resulting in placentae that share a common amniotic sac. In this case, patient D (monochorionic diamniotic) is the correct answer because the placenta is monochorionic, meaning they share a single placenta. Additionally, the diamniotic part indicates that each fetus has its own amniotic sac.

**Why Each Wrong Option is Incorrect:**

A. Fused dichorionic diamniotic placenta (patient A) indicates that the placentae are separate and have their own amniotic sacs, so these twins are not identical.
B. Circumvallate placenta (patient C) is a specific placental form and does not indicate the type of twins.

**Why Patient C is Incorrect:** Circumvallate placenta is a unique placental feature, but it doesn't determine the type of twins. The placental morphology alone cannot definitively identify whether the twins are identical or fraternal.

**Clinical Pearl:** The correct identification of twins' relationship relies on a combination of placental and ultrasound findings, as well as maternal factors such as identical twins being more likely in monozygotic mothers.

**Correct Answer:** D. Monochorionic diamniotic