Bioluminescence is well marked in

Options

A. flatworms
B. ctenophores
C. cnidarians
D. aschelminthes

Answer

### Step-by-Step Solution:

1. **Understanding Bioluminescence**:
- Bioluminescence is a natural phenomenon where living organisms produce light. The term "bio" means life, and "luminescence" refers to light emission.

2. **Identifying the Options**:
- The question provides four options: flatworm, ctenophores, cnidarians, and aschelminthes. We need to determine which of these groups exhibits bioluminescence.

3. **Analyzing Each Option**:
- **Flatworms**: Generally do not exhibit bioluminescence.
- **Ctenophores**: Known for their ability to produce light. They are often referred to as comb jellies and are well known for their bioluminescent properties.
- **Cnidarians**: Some species, like certain jellyfish, can exhibit bioluminescence, but it is not as widespread as in ctenophores.
- **Aschelminthes**: This group does not typically show bioluminescence.

4. **Conclusion**:
- Based on the analysis, ctenophores are the group that is well marked for bioluminescence. Therefore, the correct answer to the question is **ctenophores**.

### Final Answer:
Bioluminescence is well marked in **ctenophores**.

---