The correct answer is option 4: Thermal power plants. Fly ash is a byproduct of coal combustion in thermal power plants. When coal is burned in these power plants to generate electricity, the combustion process produces ash, which includes fly ash.

Option 1: Burning of agricultural waste is incorrect because the ash produced from burning agricultural waste is called agricultural waste ash, not fly ash.

Option 2: Burning of municipal solid waste is also incorrect because the ash produced from burning municipal solid waste is called municipal solid waste ash, not fly ash.

Option 3: Mining of aluminum and iron ores is incorrect because fly ash is not emitted from the mining of aluminum and iron ores.

Therefore, option 4: Thermal power plants is the correct answer as it is the main source of fly ash emissions. Fly ash can be collected, processed, and used in various applications such as making bricks.