**Answer:**

The correct option is (b) CO2.

**Reasoning:**

**1. Introduction:**
Air pollutants can have adverse effects on plants, affecting their growth, development, and overall health. However, not all air pollutants have a negative impact on plants. Among the options provided, carbon dioxide (CO2) is the air pollutant that does not have an adverse effect on plants.

**2. Adverse Effects of Air Pollutants on Plants:**
Air pollutants such as sulfur dioxide (SO2) and hydrogen cyanide (HCN) can have damaging effects on plants. These pollutants can enter plant tissues through stomata or through direct contact with the plant surface, leading to various detrimental effects.

- **SO2:** Sulfur dioxide is a byproduct of fossil fuel combustion, industrial processes, and volcanic activities. When SO2 is present in the air, it can enter plant tissues through stomata. Once inside the plant, SO2 can cause damage by disrupting cellular processes, inhibiting photosynthesis, and impairing plant growth and development. It can also lead to necrosis, chlorosis, and leaf damage.

- **HCN:** Hydrogen cyanide is a toxic gas released during combustion processes, such as biomass burning. HCN can enter plant tissues through stomata or through direct contact with the plant surface. It is toxic to plants and can inhibit enzyme activity, disrupt metabolic processes, and ultimately lead to plant death.

**3. Carbon Dioxide (CO2):**
Unlike SO2 and HCN, carbon dioxide (CO2) is not considered an air pollutant when present in normal concentrations. In fact, carbon dioxide is essential for photosynthesis, the process by which plants convert CO2 and sunlight into energy and oxygen. Plants require CO2 for their growth and development, and higher levels of CO2 can often stimulate plant growth and productivity.

- **Importance of CO2 for Plants:** Carbon dioxide is a necessary component for photosynthesis. It is absorbed by plants through stomata, and with the help of sunlight, it is converted into energy-rich carbohydrates. CO2 is a fundamental building block for plant growth, and higher concentrations of CO2 can often result in increased plant productivity and biomass.

**4. Conclusion:**
Among the given options, carbon dioxide (CO2) is not an air pollutant that has adverse effects on plants. In fact, plants require CO2 for photosynthesis and growth. On the other hand, SO2 and HCN can have detrimental effects on plants, inhibiting photosynthesis, disrupting metabolic processes, and damaging plant tissues.