The correct answer is D) precipitate scale forming salts into sludge. Scale prevention in boiler water is accomplished by adding treatment chemicals that cause the scale-forming minerals to precipitate out of the water and form a soft sludge that can be removed, rather than building up as hard scale on the boiler surfaces. This prevents the scale from adhering to the boiler and interfering with heat transfer. The other options are incorrect - adding chemicals to make the water neutral (A) or more acidic (B) does not directly address scale formation, and solidifying scale into powder (C) would not prevent it from building up on the boiler.