The National Clean Air Programme (NCAP) in India is designed to tackle air pollution and improve air quality in 102 cities by reducing particulate matter (PM) levels by 20% - 30% by 2024. In order to measure the progress made in reducing PM levels, a comparison needs to be made between the current concentrations and a base year.

The correct answer to the question is option 3: 2017. This means that the base year for comparison is 2017. The NCAP will aim to reduce PM levels in the coming years and measure progress based on the levels observed in 2017. By doing this, it will be possible to assess the effectiveness of the program in achieving the desired reduction in PM levels.

It is important to note that the base year for comparison may vary depending on the specific monitoring and data collection methods used by the NCAP. However, based on the information provided in the question, the correct answer is 2017.