Assertion :- In a myelinated nerve fibre the impulse jumps from one node of Ranvier to the other.
Reason:- Exchange of ions takes place only at node of Ranvier.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

To solve the question, we need to evaluate the assertion and the reason provided, and determine their validity and relationship.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in a myelinated nerve fiber, the impulse jumps from one node of Ranvier to another.
- This is true because myelinated fibers have segments covered by myelin sheath, which prevents ion exchange along the length of the axon. The action potential (nerve impulse) is regenerated at the nodes of Ranvier, allowing the impulse to "jump" from node to node in a process called saltatory conduction.

2. **Understanding the Reason**:
- The reason states that the exchange of ions takes place only at the node of Ranvier.
- This statement is also true. In myelinated fibers, the myelin sheath insulates the axon, and ion exchange (which is crucial for the generation of action potentials) occurs only at the nodes of Ranvier where the axon membrane is exposed.

3. **Evaluating the Relationship**:
- The reason provides a correct explanation for the assertion. The jumping of the impulse from node to node (assertion) is indeed due to the fact that ion exchange occurs only at the nodes (reason).

4. **Conclusion**:
- Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
- The correct option is: **Option 1** - Both assertion and reason are true, and the reason is the correct explanation of the assertion.