The organ of nutrition used in fetal development in the most successful mammals is the :

Options

A. mammary gland
B. pouch
C. placenta
D. marsupium

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the organ responsible for nutrition during fetal development in the most successful mammals.

2. **Identifying Key Terms**: The key terms here are "organ of nutrition," "fetal development," and "most successful mammals."

3. **Recognizing the Role of Maternal Nutrition**: In mammals, maternal nutrition is crucial for the growth and development of the fetus. This indicates that there is a specific organ involved in transferring nutrients from the mother to the fetus.

4. **Exploring Fetal Circulatory System**: The fetal circulatory system is integrated with the placenta, which is essential for the exchange of nutrients and oxygen between the mother and the fetus.

5. **Understanding the Placenta**: The placenta is an organ that develops in the uterus during pregnancy. It serves as the interface between the mother and the fetus, allowing for the transfer of nutrients, oxygen, and waste products.

6. **Connection via Umbilical Cord**: The placenta is connected to the fetus via the umbilical cord, which facilitates the delivery of nutrients and oxygen to the developing fetus.

7. **Conclusion**: Based on the above points, the organ of nutrition used in fetal development in the most successful mammals is the placenta.

**Final Answer**: The organ of nutrition used in fetal development in the most successful mammals is the **placenta**.

---