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 will analyze each option provided and determine which one represents mutualism.

### Step-by-Step Solution:

1. **Understanding Mutualism**:
- Mutualism is a type of symbiotic relationship where both partners benefit from the association.

2. **Analyzing the First Option: Ophiris and Bees**:
- Ophiris opifera, known as the bee orchid, has flowers that resemble female bees and emit a fragrance similar to that of female bees.
- Male bees are attracted to these flowers, thinking they are female bees, and in the process of attempting to mate (pseudocopulation), they transfer pollen from one flower to another.
- This results in pollination, benefiting both the bees (by providing a mating opportunity) and the orchid (by facilitating reproduction).
- **Conclusion**: This is an example of mutualism.

3. **Analyzing the Second Option: Sea Anemone and Clownfish**:
- Sea anemones have tentacles that provide protection from predators, and clownfish live among these tentacles.
- The clownfish receive protection, while the sea anemone benefits from the clownfish's presence as they help keep the area clean.
- However, this relationship is more accurately described as commensalism since the clownfish benefits while the sea anemone is neither significantly harmed nor benefited.
- **Conclusion**: This is not mutualism.

4. **Analyzing the Third Option: Cuckoo and Crow**:
- Cuckoos lay their eggs in the nests of crows, often destroying some of the crow's eggs in the process.
- This relationship is an example of brood parasitism where the cuckoo benefits at the expense of the crow.
- **Conclusion**: This is not mutualism.

5. **Analyzing the Fourth Option: Cuscuta and Hedge Plants**:
- Cuscuta, or dodder, is a parasitic plant that has lost chlorophyll and derives nutrients from host plants (hedge plants).
- This relationship is parasitic, as the cuscuta benefits while the host plant is harmed.
- **Conclusion**: This is not mutualism.

6. **Final Decision**:
- After analyzing all options, the only example of mutualism is between Ophiris and bees.

### Final Answer:
The association essential and beneficial to both partners is observed between **Ophiris and bees**.