Aquatic plants usually have a well-developed :

Options

A. vascular system
B. stomatal system
C. root system
D. aerenchyma system

Explanation

To solve the question "Aquatic plants usually have a well-developed:", we need to analyze the options provided and understand the characteristics of aquatic plants.

### Step-by-Step Solution:

1. **Understanding Aquatic Plants**:
Aquatic plants, also known as hydrophytes, are plants that have adapted to living in water. They have specific adaptations that allow them to thrive in aquatic environments.

2. **Analyzing the Options**:
- **Vascular System**: This system is responsible for the conduction of water and nutrients throughout the plant. While it is important, it is not the most distinctive feature of aquatic plants.
- **Stomatal System**: This system is involved in gas exchange through stomata. It is more relevant to terrestrial plants where gas exchange is crucial for photosynthesis.
- **Root System**: This system is primarily for anchoring the plant and absorbing nutrients from the soil. Aquatic plants often have reduced root systems since they are submerged in water.
- **Aerenchyma System**: This system consists of specialized cells that provide buoyancy and help aquatic plants float. Aerenchyma cells have large air spaces that allow for gas exchange and support the plant structure in water.

3. **Identifying the Well-Developed System**:
Among the options, the aerenchyma system is particularly well-developed in aquatic plants. This adaptation is crucial for their survival as it allows them to float and access sunlight for photosynthesis.

4. **Conclusion**:
Therefore, the correct answer to the question is **Aerenchyma System**.