A: Acid rain reduces the fertility of the soil,
R: Acid rain has a corronding effect on marble buildings.

Answer

To solve the assertion and reason question regarding acid rain, we will break down the information provided and analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states: "Acid rain reduces the fertility of the soil."
- Acid rain is formed when pollutants such as sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) react with water vapor in the atmosphere, leading to the formation of sulfuric acid (H₂SO₄) and nitric acid (HNO₃).
- These acids increase the acidity of the soil, which can harm plants and reduce soil fertility by leaching essential nutrients away from the soil.

2. **Understanding the Reason (R)**:
- The reason states: "Acid rain has a corroding effect on marble buildings."
- Marble is primarily composed of calcium carbonate (CaCO₃). When acid rain, which contains sulfuric acid, falls on marble, a chemical reaction occurs:
\[ \text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2 \]
- This reaction results in the corrosion of marble, leading to deterioration of buildings and monuments.

3. **Evaluating the Statements**:
- Both the assertion and the reason are scientifically accurate.
- The assertion correctly identifies the negative impact of acid rain on soil fertility.
- The reason correctly describes the corrosive effect of acid rain on marble buildings.

4. **Determining the Relationship**:
- While both statements are true, the reason does not provide a direct explanation for the assertion. The reduction in soil fertility is primarily due to the leaching of nutrients and not directly related to the corrosion of marble buildings.

5. **Conclusion**:
- Therefore, the correct answer is that both the assertion and the reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
**Option B**: Assertion and reason are true, but the reason is not the correct explanation of the assertion.