$ Notochord is present in vertebrates.
! Notochord is always formed of bones.

Options

A. If both A and R are correct and R is correct explanation of A
B. If both A and R are correct but R is not correct explanation of A
C. If A is correct, R is wrong
D. If both A and R are wrong.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Notochord is present in vertebrates."
- The notochord is indeed a characteristic feature of vertebrates, serving as a structural support during the early stages of development.

2. **Understanding the Reason**: The reason given is "Notochord is always formed of bones."
- This statement is incorrect. The notochord is primarily made up of cartilage, not bones.

3. **Identifying the Nature of the Notochord**:
- The notochord is a rod-like structure that is derived from the mesoderm layer of the embryo. It is located on the dorsal side of the developing embryo.

4. **Conclusion**:
- Since the assertion is true (notochord is present in vertebrates) and the reason is false (notochord is not formed of bones), we can conclude that the assertion is correct while the reason is incorrect.

5. **Final Answer**:
- The correct option is that the assertion is correct and the reason is wrong.