The primary source of pollution in the Yamuna River in Delhi is the dumping of domestic waste. Due to rapid urbanization, untreated sewage, household waste, and industrial effluents are directly released into the river. This has led to severe water pollution, with high levels of toxins and waste in the water. While the use of pesticides and fertilizers does contribute to pollution in agricultural areas, the dumping of domestic waste is the most significant factor in polluting the Yamuna in Delhi.
Thus, the correct option is (B) Dumping of domestic waste.