According to Air Quality Index (AQI) rankings for 2018, which city was identified as the most polluted city in the world?

Introduction / Context:
Air pollution is a major environmental and health issue, and many organizations publish rankings of cities based on air quality indices. Competitive exams often include questions about which cities top such lists, especially when an Indian city appears at the top. This question refers to the Air Quality Index rankings for 2018 and asks which city was identified as the most polluted at that time.

Given Data / Assumptions:

- The year of reference is 2018.- The parameter is the Air Quality Index and overall pollution level.- Options are Sydney, New Delhi, Mumbai, and Islamabad.- We assume standard exam oriented summaries of 2018 AQI reports.

Concept / Approach:
Several reports around 2018 highlighted the severe air pollution in New Delhi, especially during winter months. In many exam oriented question banks, New Delhi is listed as the most polluted city in the world according to an Air Quality Index ranking for 2018. The approach, therefore, is to recall these widely cited general awareness facts and match them with the given choices. Among the four cities offered, New Delhi is consistently presented as the most polluted in that specific context.

Step-by-Step Solution:
1. Recall news and reports from 2018 and early 2019 that frequently mentioned New Delhi as ranking among the top polluted cities globally.2. Recognize that Sydney in Australia and Islamabad in Pakistan were not commonly reported as the single most polluted city by AQI for that year.3. Though Mumbai also has serious air pollution, it was New Delhi that featured most prominently in global rankings and headlines for extreme pollution levels.4. Compare each option with this remembered pattern and see that only New Delhi matches the repeated general awareness statements.5. Select New Delhi as the correct answer.

Verification / Alternative check:
You can cross check this by remembering specific events such as schools closing, air emergency declarations, and repeated public debates about smog in New Delhi. International bodies and media outlets used AQI data to show that New Delhi regularly recorded extremely high particulate matter levels. These stories were far more frequent for New Delhi than for Sydney, Mumbai, or Islamabad, which supports its position in many exam oriented summaries as the most polluted city according to the 2018 AQI rankings referred to in this question.

Why Other Options Are Wrong:
- Sydney: This Australian city generally has better air quality and is not listed as the most polluted city in world AQI rankings for that year.- Mumbai: While polluted, Mumbai did not overtake New Delhi in the specific AQI based ranking being referred to in common question banks.- Islamabad: Pakistan does have pollution issues, but Islamabad is not the city most often singled out as the most polluted in the global 2018 AQI context.

Common Pitfalls:
Students sometimes overthink the question and suspect a trick, especially because more than one city listed has pollution problems. Another mistake is to confuse different years or different reports; some rankings focus on countries or on long term averages rather than a single AQI based city list. To avoid confusion, pay close attention to the year mentioned and to the exact phrasing “According to Air Quality Index 2018,” which matches how many exam sources describe New Delhi in that period.

Final Answer:
The correct option is New Delhi, which is commonly identified as the most polluted city in the world according to Air Quality Index based rankings for 2018 in general awareness material.