Of the following groups of organisms, which is the most evolutionarily advanced, with more complex body structure ?

Options

A. Roundworms
B. Cnidaria
C. Sponges
D. Flatworms

Explanation

To determine which group of organisms is the most evolutionarily advanced with a more complex body structure, we will analyze the four options provided: roundworms, cnidaria, sponges, and flatworms.

### Step-by-Step Solution:

1. **Identify the Groups**: The four groups of organisms we need to evaluate are:
- Roundworms (Phylum Nematoda)
- Cnidaria (Phylum Cnidaria)
- Sponges (Phylum Porifera)
- Flatworms (Phylum Platyhelminthes)

2. **Analyze Roundworms**:
- Roundworms have a **complete digestive system**, which means they possess two openings: a mouth and an anus. This allows for a more efficient digestion process.
- They exhibit **bilateral symmetry**, which is a characteristic of more complex organisms.

3. **Analyze Cnidaria**:
- Cnidarians have an **incomplete digestive system**, meaning they only have one opening for both ingestion and egestion.
- They display **radial symmetry**, which is typical of simpler organisms.

4. **Analyze Sponges**:
- Sponges lack a true digestive system altogether; they filter feed through their porous bodies.
- They exhibit **asymmetrical body plans**, indicating a lower level of complexity.

5. **Analyze Flatworms**:
- Flatworms also have an **incomplete digestive system**, with only one opening (mouth).
- They possess **bilateral symmetry**, but their overall body structure is simpler compared to roundworms.

6. **Compare the Complexity**:
- Roundworms are the only group with a complete digestive system and bilateral symmetry, indicating a higher level of complexity.
- Cnidaria, sponges, and flatworms all have simpler body structures and incomplete digestive systems.

7. **Conclusion**:
- Based on the analysis, **roundworms** are the most evolutionarily advanced group among the options provided due to their complete digestive system and bilateral symmetry.

### Final Answer:
The most evolutionarily advanced group with a more complex body structure is **roundworms**.