Organ system level of organisation is observed in

Options

A. Chordates
B. Annelids
C. Mollusca
D. All of these

Answer

To determine which organisms exhibit the organ system level of organization, we can analyze the options provided: caudates, annelids, mollusks, or all of these.

### Step-by-Step Solution:

1. **Understand Organ System Level of Organization**:
- The organ system level of organization refers to a complex arrangement where different organs work together to perform specific physiological functions. This includes systems such as the circulatory system, respiratory system, digestive system, etc.

2. **Examine the Options**:
- **Caudates (Chordates)**: This group includes vertebrates like mammals, birds, reptiles, amphibians, and fish. Chordates have a well-defined organ system level of organization, with specialized organs forming systems that perform vital functions.
- **Annelids**: This group includes segmented worms like earthworms and leeches. Annelids also exhibit an organ system level of organization, with various organs working together for functions such as digestion and circulation.
- **Mollusks**: This group includes snails, clams, and squids. Mollusks possess an organ system level of organization as well, with specialized organs for processes like feeding, respiration, and reproduction.

3. **Conclusion**:
- Since all three groups (caudates, annelids, and mollusks) display the organ system level of organization, the correct answer is "all of these".

### Final Answer:
All of these.