Choanocytes are present in

Options

A. Physalia
B. Sycon
C. Pleurobrachia
D. Echinus

Explanation

**Step-by-Step Solution:**

1. **Understanding Choanocytes**:
- Choanocytes are specialized cells that are flagellated and are primarily found in sponges, which belong to the phylum Porifera. These cells play a crucial role in the feeding mechanism of sponges by creating water currents and trapping food particles.

2. **Identifying the Phylum**:
- The question asks where choanocytes are present. Since choanocytes are characteristic of sponges, we need to look for options that belong to the phylum Porifera.

3. **Analyzing the Options**:
- The options provided are:
- a) Physalia
- b) Cyclidia
- c) Pleurobrachia
- d) Echinus
- Now, we need to identify which of these belongs to the phylum Porifera.

4. **Evaluating Each Option**:
- **Physalia**: This organism belongs to the phylum Cnidaria (Coelenterata) and does not have choanocytes.
- **Cyclidia**: This is a sponge and belongs to the phylum Porifera, which means it contains choanocytes.
- **Pleurobrachia**: This organism is a comb jelly and belongs to the phylum Ctenophora, not Porifera.
- **Echinus**: This is a sea urchin and belongs to the phylum Echinodermata, which also does not have choanocytes.

5. **Conclusion**:
- Based on the analysis, the only option that contains choanocytes is **Cyclidia**. Therefore, the correct answer is **Cyclidia**.