Explanation:
The National Clean Air Programme (NCAP) is a comprehensive program launched by the Government of India in January 2019 to tackle the growing problem of air pollution in the country. The program aims to reduce particulate matter (PM) in at least 102 cities of India by 20% - 30% by 2024.

Base Year for Comparison:
The base year for the comparison of concentration, i.e., the year against which the reduction in particulate matter (PM) levels will be measured, is 2017. This means that the concentration of PM in the selected cities will be measured in 2017, and the target is to reduce this concentration by 20% - 30% by 2024.

Reason for Choosing 2017 as the Base Year:
The selection of 2017 as the base year for comparison is based on several factors:

1. Availability of Data: The government collects data on air pollution levels regularly from various monitoring stations across the country. The data for 2017 is readily available and can be used as a baseline to measure the progress in reducing particulate matter (PM) levels.

2. Representation of Current Situation: The year 2017 is recent enough to represent the current situation of air pollution in the selected cities. Using a more recent year ensures that the baseline data accurately reflects the severity of the problem and the need for intervention.

3. Sufficient Time for Action: By choosing 2017 as the base year, the government allows sufficient time for implementing measures to reduce air pollution. The target of reducing PM levels by 20% - 30% by 2024 gives a timeframe of seven years for the implementation of various interventions and policies.

4. Long-term Comparison: Selecting 2017 as the base year allows for a long-term comparison of air pollution levels. It provides a benchmark against which future progress can be measured and helps in evaluating the effectiveness of the National Clean Air Programme (NCAP) over time.

In conclusion, the base year for the comparison of concentration in the National Clean Air Programme (NCAP) is 2017. This choice ensures the availability of data, represents the current situation, allows sufficient time for action, and enables long-term comparison of air pollution levels.