The main cause for abundant coliform bacteria in the river Ganga is (a) disposal of unburnt corpses into water.
Explanation: Coliform bacteria are commonly found in human feces, so their presence in a water body indicates fecal contamination. Unburnt corpses can significantly contaminate the river with these bacteria when disposed of improperly.
Why other options are incorrect:
(b) discharge of effluents from electroplating industries:
While industrial effluents can pollute rivers with various chemicals, coliform bacteria are primarily associated with fecal matter. Electroplating processes typically involve different types of pollutants not directly related to coliform bacteria.
(c) washing of clothes:
Washing clothes, while contributing to water pollution with detergents and other chemicals, is not the primary source of coliform bacteria. Coliform bacteria are primarily found in human and animal feces.
(d) immersion of ashes:
The immersion of ashes, although a cultural practice, is not considered a significant contributor to coliform bacteria contamination in the river Ganga. Ashes do not contain large amounts of coliform bacteria.