The Yamuna river in Delhi faces serious pollution primarily due to the dumping of domestic sewage and waste. A significant portion of untreated domestic wastewater is discharged into the river, causing high levels of organic pollution and water quality deterioration.
Flood water spreading (A): Contributes little to pollution compared to direct waste dumping.
Dumping of domestic waste (B): Major cause of pollution, leading to depletion of dissolved oxygen and harming aquatic life.
Use of pesticides (C) and fertilizers (D): These affect water bodies mainly through agricultural runoff, less relevant in Delhi's urban pollution.
So, the correct answer is (B): Dumping of domestic waste.