According to a World Health Organization (WHO) study, which Indian city was identified as the most polluted city in the world?

Introduction / Context:
Air pollution is a major environmental and public health issue, especially in rapidly growing cities. The World Health Organization (WHO) periodically publishes studies and databases on air quality around the world. In one widely discussed report, an Indian city was highlighted as the most polluted city in the world based on particulate matter concentration. This question tests whether candidates remember which city was identified in that WHO study.

Given Data / Assumptions:

• The study was conducted by the World Health Organization (WHO).
• It ranked cities based on pollution levels, usually particulate matter such as PM2.5 or PM10.
• The question is specifically about the most polluted city in the world in that report.
• Options are four major Indian cities: Delhi, Mumbai, Pune, and Chennai.

Concept / Approach:
WHO's air quality reports have repeatedly drawn attention to very high particulate levels in some Indian cities. Media coverage and exam capsules focused strongly on Delhi, which has regularly figured among the worst performers. By recalling those news items and environment sections of current affairs notes, candidates can eliminate less likely options and select the correct city.

Step-by-Step Solution:
Step 1: Recall that headlines often highlighted Delhi's severe smog, especially in winter, with phrases such as emergency pollution levels. Step 2: WHO reports and many news articles specifically mentioned Delhi as being at or near the top of global rankings for poor air quality in a given year. Step 3: Compare with the other options. Mumbai, Pune, and Chennai do have pollution problems, but they are generally not cited as the number one globally in those WHO lists. Step 4: Based on this, conclude that Delhi was the city labelled the most polluted in the world in that particular WHO study. Step 5: Mark Delhi as the correct answer.

Verification / Alternative check:
A verification route is to recall environment and geography exam notes that summarise WHO air quality rankings. These notes frequently state that Delhi topped the list of most polluted cities by PM2.5 concentration in an earlier WHO database, which matches the wording of this question. Cross referencing multiple exam preparation sources confirms the selection of Delhi.

Why Other Options Are Wrong:
Mumbai: While Mumbai faces pollution from traffic and industry, it has not been the headline city for the worst global ranking in the specific WHO study mentioned, so this option is incorrect.
Pune: Pune is usually considered somewhat better in air quality relative to the most polluted metros, so it does not match the description given in the question.
Chennai: Though Chennai has its own environmental challenges, it was not singled out as the most polluted city in the World Health Organization's cited study, making this option wrong here.

Common Pitfalls:
A typical pitfall is to overgeneralise and think that any large Indian city might be the answer, especially if one has heard about general pollution problems there. Another mistake is confusing different years and different reports from WHO or other agencies. Candidates should always link the specific question wording, such as "most polluted city in the world", with the city that dominated media coverage in that exact context.

Final Answer:
In the World Health Organization study referred to in the question, the Indian city identified as the most polluted city in the world was Delhi.