Reproductive system with external fertilization are most common in

Options

A. terrestrial animals
B. populations with many more males than females
C. animals that are widely dispersed
D. aquatic animals

Answer

To answer the question "Reproductive systems with external fertilization are most common in," we will analyze the options provided and determine which is the most appropriate.

### Step-by-Step Solution:

1. **Understand the Concept of External Fertilization**:
- External fertilization is the process where the male and female gametes (sperm and egg) fuse outside the body of the female organism. This typically requires a medium, such as water, for the gametes to meet and fertilize.

2. **Identify the Characteristics of External Fertilization**:
- External fertilization is common in environments where the gametes can be released into a medium that allows for their meeting. This is often seen in aquatic environments where water serves as the medium.

3. **Evaluate the Options**:
- **Terrestrial Animals**: These animals usually have internal fertilization due to the challenges of gamete survival outside the body in a terrestrial environment.
- **Populations with Many More Males than Females**: This does not directly relate to the type of fertilization but rather to population dynamics.
- **Animals that are Widely Dispersed**: While some widely dispersed animals may have external fertilization, it is not a defining characteristic.
- **Aquatic Animals**: These animals are most commonly associated with external fertilization because they release their gametes into water, which facilitates fertilization.

4. **Conclusion**:
- Based on the analysis, the correct answer is **Aquatic Animals**. They are the group where external fertilization is most prevalent due to the necessity of a medium (water) for the gametes to meet.

### Final Answer:
Reproductive systems with external fertilization are most common in **aquatic animals**.

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