AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
(B) Hot water drained from factories and thermal plants before cooling
Thermal pollution occurs when hot water from factories and thermal plants is drained into rivers and ponds before cooling.
The passage explicitly states: "Thermal pollution of water occurs when hot water from factories and thermal plants is drained into rivers and ponds before cooling." Option B directly matches this definition. Fly ash causes solid waste/water pollution (not thermal), toxic gas leaks cause air pollution, and slag is a solid waste — none of these cause thermal pollution.