Assertion : Endomycorrhiza of forest trees contribute to the efficient nutrient cycling in tropical forest ecosystem.
Reason : The fungi that form mycorrhizal associations with plants make nutrient ions available to them.

Options

A. if both are true with reason being correct explanation
B. both are true but reason is not correct explanation
C. assertion is true but reason is wrong
D. both are wrong

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Endomycorrhiza of forest trees contribute to the efficient nutrient cycling in tropical forest ecosystems."
- Endomycorrhiza refers to a symbiotic association between fungi and the roots of higher plants, particularly in forest ecosystems.

2. **Understanding the Reason**:
- The reason provided is that "The fungi that form mycorrhizal associations with plants make nutrient ions available to them."
- This means that the fungi help in the absorption of nutrients from the soil, which are then made available to the plants.

3. **Analyzing the Relationship**:
- Endomycorrhiza fungi invade the root cells of plants and extend their hyphae into the soil, which increases the surface area for nutrient absorption.
- They are particularly effective in enhancing the uptake of phosphate ions, which are crucial for plant growth.

4. **Conclusion**:
- Since both the assertion and the reason are true, and the reason correctly explains why endomycorrhiza contributes to nutrient cycling, we can conclude that:
- **Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.**

5. **Final Answer**:
- The correct option is **A**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.