Mycorrhizae are :

Options

A. asexual reproductive structures formed by lichens
B. mycelium that forms fairy rings
C. compact, dikaryotic hyphae that form a basidiocarp
D. mutualistic associations between plant roots and fungi

Explanation

To answer the question "Mycorrhizae are:", we need to analyze the given options and understand the definition of mycorrhizae.

### Step-by-Step Solution:

1. **Understand the Definition of Mycorrhizae**:
Mycorrhizae are a type of symbiotic association between the roots of higher plants and fungi. This relationship is beneficial for both organisms involved.

2. **Analyze the Options**:
- **Option A**: Asexual reproductive structures formed by lichens.
- This is incorrect because mycorrhizae are not related to lichens or their reproductive structures.
- **Option B**: Mycelium that forms fairy rings.
- This is also incorrect. Fairy rings are a phenomenon caused by certain fungi but do not define mycorrhizae.
- **Option C**: Compact dikaryotic hyphae that form a basidiocarp.
- This option is incorrect as well. Basidiocarps are structures formed by certain fungi (basidiomycetes) and are not related to mycorrhizae.
- **Option D**: Mutualistic associations between plant roots and fungi.
- This is the correct answer. Mycorrhizae represent a mutualistic relationship where fungi provide nutrients and water to the plants, while plants provide carbohydrates to the fungi.

3. **Conclusion**:
Based on the analysis, the correct answer is **Option D: mutualistic associations between plant roots and fungi**.

### Final Answer:
Mycorrhizae are mutualistic associations between plant roots and fungi.

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