**Core Concept**
The question tests knowledge of indicators of air pollution. Air pollution is a complex mixture of various pollutants, including gases, particulate matter, and other substances that can harm human health and the environment. Indicators of air pollution are essential for monitoring and controlling pollution levels.

**Why the Correct Answer is Right**
The correct answer is **D. Smoking index**. The smoking index is a measure of the level of particulate matter in the air due to domestic cooking and biomass burning, which is not directly related to air pollution from industrial or vehicular sources. It does not specifically indicate the presence of air pollutants like carbon monoxide (CO), sulfur dioxide (SO2), or particulate matter (PM) that are typically associated with air pollution.

**Why Each Wrong Option is Incorrect**
**Option A:** CO is a colorless, odorless gas that is a potent air pollutant, especially in urban areas with high levels of traffic. It is a byproduct of incomplete combustion of fossil fuels and can cause respiratory problems and other health issues.

**Option B:** SO2 is a toxic gas that is released into the atmosphere through industrial activities, fossil fuel combustion, and volcanic eruptions. It contributes to acid rain, respiratory problems, and other health issues.

**Option C:** The soiling index is a measure of the level of particulate matter in the air, which can be caused by various sources, including industrial activities, vehicular emissions, and natural events like dust storms. It is a valid indicator of air pollution.

**Clinical Pearl / High-Yield Fact**
A key indicator of air pollution is the particulate matter (PM) level, which can be measured using various methods, including the soiling index. High levels of PM can cause respiratory problems, cardiovascular disease, and other health issues.

**Correct Answer:**
✓ Correct Answer: D. Smoking index