This secondary pollutant has caused significant damage to the beauty of historical monuments like the Taj Mahal and Red fort. Identify the correct match of the responsible pollutant and its characteristics feature.

Options

A. Acid rain - ratio of `H_2SO_4 and HNO_3` is 3 : 7
B. Photochemical Smog - `PAN +O_3+` Nitrogen oxides
C. Acid rain - pH of `lt 5.6`
D. Sulphur smog - formed from vapour of `H_2SO_4`

Answer

To solve the question regarding the secondary pollutant that has caused damage to historical monuments like the Taj Mahal and Red Fort, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Concept of Secondary Pollutants**:
- Secondary pollutants are not directly emitted into the atmosphere. Instead, they form when primary pollutants undergo chemical reactions in the atmosphere.

**Hint**: Recall the difference between primary and secondary pollutants. Primary pollutants are emitted directly, while secondary pollutants form from reactions involving primary pollutants.

2. **Identifying the Relevant Pollutants**:
- Common primary pollutants include carbon dioxide, nitrogen oxides, and sulfur oxides. These substances can react with water vapor in the atmosphere.

**Hint**: Think about the sources of pollution and which gases are released from human activities like burning fossil fuels.

3. **Recognizing the Formation of Acid Rain**:
- When nitrogen oxides and sulfur oxides react with water vapor, they can form acids such as sulfuric acid and nitric acid. This process leads to the formation of acid rain.

**Hint**: Consider how rainwater, which is typically neutral, can become acidic due to the presence of certain gases.

4. **Understanding the Impact on Monuments**:
- Acid rain has a pH lower than 5.6, which is harmful to structures made of calcium carbonate, such as marble. The Taj Mahal, made of white marble, is particularly susceptible to corrosion from acid rain.

**Hint**: Reflect on the materials used in historical monuments and how they might react with acidic substances.

5. **Concluding with the Correct Match**:
- The correct match for the secondary pollutant responsible for damaging historical monuments like the Taj Mahal and Red Fort is acid rain, characterized by its low pH due to the presence of sulfuric and nitric acids formed from nitrogen and sulfur oxides.

**Hint**: Focus on the specific characteristics of acid rain that make it harmful to monuments.

### Final Answer:
The correct answer is **acid rain**, which is a secondary pollutant with a pH less than 5.6, caused by the reaction of nitrogen and sulfur oxides with water vapor in the atmosphere.