The correct answer is D) Steam heating coils are usually the header type and, if possible, vertically installed. This is because the header type steam heating coils are more efficient and easier to install vertically, as this allows for proper drainage and circulation of the steam. Vertically installed coils also help prevent air pockets from forming, which can reduce the efficiency of the heating system. The other options are incorrect because serpentine coils are less common for steam heating, and horizontal installation is generally less desirable as it can make it more difficult to properly drain the system and maintain efficient steam circulation.