In Brood parasitism :

Options

A. Cuckoo bird lay its eggs in the nest of its host's nest
B. Cusuta growing on host plant
C. Lice on human
D. Clown fish that lives among sea anemone

Answer

### Step-by-Step Solution:

1. **Understanding Brood Parasitism**:
- Brood parasitism is a specific type of parasitism where one species (the parasite) relies on another species (the host) to raise its young. The parasite lays its eggs in the nest of the host, and the host then unknowingly incubates and raises the parasite's offspring.

2. **Identifying the Cuckoo Bird's Behavior**:
- The cuckoo bird is a well-known example of a brood parasite. It lays its eggs in the nests of other bird species. The host birds then take care of the cuckoo eggs as if they were their own, often at the expense of their own offspring.

3. **Analyzing the Options**:
- **Option A**: Cuckoo bird lays its egg in the nest of its host. This aligns with the definition of brood parasitism.
- **Option B**: Cuscuta (a parasitic plant) growing on a host plant does not represent brood parasitism but rather a different type of parasitism.
- **Option C**: Lice on humans is a form of parasitism but not brood parasitism.
- **Option D**: Clownfish living among sea anemones is a mutualistic relationship, not parasitism.

4. **Conclusion**:
- Based on the definitions and behaviors described, the correct answer is **Option A**: The cuckoo bird lays its egg in the nest of its host.

### Final Answer:
**A. Cuckoo bird lays its egg in the nest of its host.**

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