Assertion :- Balanoglossus and Branchiostoma are triploblastic, eucoelomate and having bilateral symmetry.
Reason :- Both are exclusively marine and having notochord.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are false.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Balanoglossus and Branchiostoma are triploblastic, eucoelomate, and having bilateral symmetry."
- **Triploblastic**: This means that these organisms have three germ layers: ectoderm, mesoderm, and endoderm.
- **Eucoelomate**: This indicates that they have a true coelom, which is a body cavity completely lined with mesoderm.
- **Bilateral Symmetry**: This means that their body can be divided into two mirror-image halves along one plane.

**Conclusion**: The assertion is correct for both Balanoglossus and Branchiostoma.

2. **Understanding the Reason**:
- The reason states that "Both are exclusively marine and having notochord."
- While it is true that both organisms are marine, the statement about the notochord is incorrect when referring to Balanoglossus.
- **Balanoglossus**: This organism does not possess a notochord in the same way that vertebrates do.
- **Branchiostoma**: This organism does have a notochord, which is a characteristic feature of cephalochordates.

**Conclusion**: The reason is partially true (both are marine) but incorrect regarding the presence of the notochord in Balanoglossus.

3. **Evaluating the Options**:
- **Option 1**: Both assertion and reason are true, and the reason is the correct explanation of the assertion. (Incorrect)
- **Option 2**: Both assertion and reason are true, but the reason is not the correct explanation of the assertion. (Incorrect)
- **Option 3**: Reason is true, and assertion is false. (Incorrect)
- **Option 4**: Assertion is true, and reason is false. (Correct)

### Final Answer:
The correct option is **Option 4**: Assertion is true, and reason is false.

---