Answer : 2. "Varanasi and Prayagraj"
Explanation :

  Varanasi and Prayagraj of Uttar Pradesh have been selected as the cleanest Ganga cities. MHOW Cantonment Board awarded as the cleanest cantonment town in Madhya Pradesh. Swachh Survekshan, conducted by the Union Ministry of Housing and Urban Affairs (MoHUA) under the Swachh Bharat Urban Mission since 2016, is the world's largest survey on urban sanitation and hygiene.