Explanation:

This question asks about the world's most polluted city as per the World Air Quality Report 2020, which is a factual query based on environmental data. - **Option (A) New Delhi:** This is the correct answer. According to the IQAir World Air Quality Report 2020, which compiles and analyzes PM2.5 data from thousands of cities globally, New Delhi was indeed ranked as the most polluted capital city in the world for the third consecutive year. The report highlighted the significantly alarming levels of particulate matter in the city's ambient air. - **Option (B) Beijing:** Beijing has historically been known for severe air pollution. However, due to substantial government efforts and policy implementations, its air quality has shown considerable improvement in recent years. In the 2020 report, Beijing's air quality had improved compared to previous years, and it was not ranked as the world's most polluted city. - **Option (C) Dhaka:** Dhaka, the capital of Bangladesh, consistently ranks among the most polluted cities globally and frequently appears in the top lists. While it faces severe air quality challenges, in the 2020 report, New Delhi surpassed it to be declared the most polluted capital city. - **Option (D) Colombo:** Colombo, the capital of Sri Lanka, generally experiences moderate to high levels of air pollution. However, its pollution levels typically do not reach the extreme concentrations observed in cities like New Delhi or Dhaka, and it was not declared the world's most polluted city in the 2020 report.\nTherefore, based on the IQAir World Air Quality Report 2020, New Delhi was identified as the world's most polluted capital city.