The correct answer is option 2, the Yamuna. The Yamuna River in India has been referred to as the open sewer due to the alarming levels of pollution and waste that it carries. The river is heavily contaminated with untreated sewage, industrial waste, and other pollutants, which has led to severe environmental degradation and health problems for those living along its banks. The pollution of the Yamuna is primarily caused by the discharge of untreated sewage from cities and towns along its course, as well as industrial effluents. Efforts have been made to clean up the river, but the problem persists and requires ongoing attention and intervention.