Assertion (A): When small amount of acid or base is added to pure water, its `pH` undergoes a change.
Reason (R) : Addition of an acid or a basic increases the degree of ionisation of water.

Options

A. If both (A) and (R) are correc, and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.

Explanation

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided:

**Assertion (A)**: When a small amount of acid or base is added to pure water, its pH undergoes a change.

**Reason (R)**: Addition of an acid or a base increases the degree of ionization of water.

### Step-by-Step Solution:

1. **Understanding pH and Ionization of Water**:
- Pure water has a neutral pH of 7 at 25°C (298 K). This is due to the self-ionization of water, where water molecules dissociate into hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) in equal concentrations.

2. **Effect of Adding Acid or Base**:
- When a small amount of acid (like HCl) is added to water, it dissociates to produce more H₃O⁺ ions. This increases the concentration of hydronium ions, leading to a decrease in pH (making the solution acidic).
- Conversely, when a small amount of base (like NaOH) is added, it dissociates to produce more OH⁻ ions. This increases the concentration of hydroxide ions, leading to an increase in pH (making the solution basic).

3. **Conclusion on Assertion (A)**:
- Since the addition of either an acid or a base changes the concentration of H₃O⁺ or OH⁻ ions, it indeed causes a change in the pH of the water. Therefore, assertion (A) is correct.

4. **Analyzing the Reason (R)**:
- The reason states that the addition of an acid or base increases the degree of ionization of water. However, the degree of ionization of water remains constant regardless of the addition of acids or bases. The concentrations of H₃O⁺ and OH⁻ ions change, but the degree of ionization (which is a measure of how much water dissociates into ions) does not change.

5. **Conclusion on Reason (R)**:
- Since the degree of ionization of water does not change with the addition of acids or bases, reason (R) is incorrect.

6. **Final Evaluation**:
- Both assertion (A) is correct, but reason (R) is incorrect. Thus, the correct option is that assertion (A) is true, but reason (R) is false.

### Final Answer:
The correct option is **Option 3**: A is correct but R is incorrect.