**Core Concept**
The soiling index is a quantitative measure used to assess the level of particulate matter in the air. It is an indicator of air pollution, specifically particulate matter (PM) that is suspended in the air.

**Why the Correct Answer is Right**
The soiling index is calculated by observing the rate at which a black cloth or surface becomes soiled by particulate matter in the air. This is a simple, low-cost method for monitoring air pollution levels, particularly in areas where more sophisticated equipment is not available. The soiling index is often used in conjunction with other methods, such as gravimetric analysis, to provide a more comprehensive understanding of air quality.

**Why Each Wrong Option is Incorrect**
**Option B:** Water pollution is typically measured using different indices, such as the Biochemical Oxygen Demand (BOD) or the Chemical Oxygen Demand (COD), which assess the level of organic or inorganic pollutants in water.
**Option C:** Faecal contamination is often measured using indices such as the E. coli count or the Total Coliform count, which assess the presence of fecal coliform bacteria in water or on surfaces.
**Option D:** Milk contamination is typically measured using indices such as the Standard Plate Count (SPC) or the Most Probable Number (MPN), which assess the presence of microorganisms in milk.

**Clinical Pearl / High-Yield Fact**
The soiling index is a useful tool for monitoring air pollution levels in urban areas, particularly in developing countries where more sophisticated equipment may not be available. It can also be used to identify areas with high levels of particulate matter, which can have serious health implications.

**✓ Correct Answer: A. Air pollution. The soiling index is a measure used to assess the level of particulate matter in the air.**