The correct answer is: A. Fertilizer.
Dried sewage after treatment is a good source of nutrients, such as nitrogen and phosphorus, which can be used to fertilize crops. It is also a good source of organic matter, which can help to improve soil structure and drainage.
Option B is incorrect because dried sewage is not a strong enough material to be used as a building material. It is also not durable enough to withstand the elements.
Option C is incorrect because dried sewage does not have the properties necessary to be used as a chemical for lowering B.O.D.
Option D is incorrect because dried sewage is not a good base material for paints. It is not smooth enough and does not have the desired properties.