Answer : 3. "इंदौर"
Explanation :

Union Minister for Environment, Forest and Climate Change Bhupendra Yadav announced the results of Swachh Vayu Survekshan 2023 (Clean Air Survey). Indore, Madhya Pradesh claimed the top spot among cities with a population of more than one million. The survey, in its second year, covered 130 cities identified under the National Clean Air Programme. Agra, Uttar Pradesh secured the second position, followed by Thane, Maharashtra at the third position.