To ensure effectiveness of reproduction in mammals, What is needed Or Which of the following happens

Options

A. Formation of yolk sac
B. Retention of yolk sac
C. Reduced number of egg
D. Formation of placenta

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks what is needed to ensure effective reproduction in mammals. This implies looking for a biological structure or process that plays a crucial role in mammalian reproduction.

2. **Identifying Options**: The options provided include:
- Formation of yolk sac
- Retention of yolk sac
- Reduced number of eggs
- Formation of placenta

3. **Analyzing Each Option**:
- **Formation of Yolk Sac**: While the yolk sac is important in some species (like reptiles), it is not a primary feature in most mammals.
- **Retention of Yolk Sac**: Similar to the first option, retention of the yolk sac is not significant in mammalian reproduction.
- **Reduced Number of Eggs**: Mammals typically produce fewer eggs compared to other vertebrates, but this alone does not ensure effective reproduction.
- **Formation of Placenta**: The placenta is a vital organ in most mammals that facilitates the exchange of nutrients and waste between the mother and the developing fetus.

4. **Conclusion**: The most critical factor for effective reproduction in mammals is the formation of the placenta. It allows for the proper development of the fetus by providing necessary nutrients and removing waste.

5. **Final Answer**: Therefore, the correct answer is **formation of placenta**.