**Core Concept**
The transfer of oxygen and nutrients from the mother to the fetus is essential for fetal growth and development. This process occurs through a specialized maternal-fetal interface, where the mother's blood comes into close contact with the fetus's blood.

**Why the Correct Answer is Right**
The correct answer is the **umbilical vein**. The umbilical vein carries oxygenated, nutrient-rich blood from the placenta to the fetus. This blood is rich in oxygen, glucose, and other essential nutrients that support fetal growth and development. The umbilical vein is a vital part of the umbilical cord, which connects the fetus to the placenta. The blood in the umbilical vein is under low pressure, allowing it to flow easily from the placenta to the fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** The **umbilical artery** carries deoxygenated, nutrient-poor blood from the fetus to the placenta. This blood is rich in waste products and carbon dioxide, which are exchanged for oxygen and nutrients in the placenta.
**Option B:** The **ductus venosus** is a shunt that allows oxygenated blood from the umbilical vein to bypass the liver and flow directly to the fetus's heart. It is not the primary vessel responsible for carrying oxygenated blood to the fetus.
**Option C:** The **fetal heart** is responsible for pumping blood throughout the fetus's body, but it is not the vessel that carries oxygenated blood from the placenta to the fetus.

**Clinical Pearl / High-Yield Fact**
The umbilical cord is a vital structure that connects the fetus to the placenta, allowing for the exchange of oxygen, nutrients, and waste products. It is essential to note that the umbilical cord is made up of two umbilical arteries and one umbilical vein.

**Correct Answer: D. Umbilical vein**