An association essential and beneficial to both the partners is observed between :

Options

A. Ophrys and bees
B. Sea anemone and clownfish
C. Cuckoo and crow
D. Cuscuta and hedge plants

Answer

To solve the question regarding the association that is essential and beneficial to both partners, we can analyze the options provided one by one.

### Step-by-Step Solution:

1. **Understanding Mutualism**:
- Mutualism is a type of symbiotic relationship where both partners benefit from the interaction. This is the key concept we need to identify in the options given.

2. **Analyzing Option A: Ophiris and Bees**:
- Ophiris is a type of Mediterranean orchid that attracts male bees by mimicking the appearance of female bees. When male bees attempt to mate with the flower (a process known as pseudocopulation), they inadvertently pick up pollen from the flower and transfer it to other flowers, facilitating pollination.
- This interaction is essential for the reproduction of the orchid and provides the bees with a mating opportunity, making it a clear example of mutualism.

3. **Analyzing Option B: Sea Anemone and Clownfish**:
- In this relationship, clownfish live among the stinging tentacles of sea anemones. The clownfish gain protection from predators, while the sea anemone benefits from the clownfish’s presence, which can help deter other predators. However, the clownfish does not provide essential benefits to the anemone, making this more of a commensal relationship rather than mutualism.

4. **Analyzing Option C: Cuckoo and Crow**:
- Cuckoos are known for brood parasitism, where they lay their eggs in the nests of other birds, like crows. The crow raises the cuckoo's young at the expense of its own. This is a clear example of parasitism, where one species benefits at the expense of another.

5. **Analyzing Option D: Cascata and Hedge Plants**:
- Cascata is a parasitic plant that derives its nutrients from hedge plants. This relationship harms the host plant while benefiting the parasitic plant, which is another example of parasitism.

6. **Conclusion**:
- After analyzing all the options, we find that the only example of mutualism, where both partners benefit and the relationship is essential for their survival, is found in **Option A: Ophiris and Bees**.

### Final Answer:
The correct answer is **A. Ophiris and Bees**.

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