The correct answer is C) Steam heating coils are usually the serpentine type and horizontally installed. The reasoning behind this is that steam heating coils in air conditioning systems are typically designed as a serpentine coil configuration, which is more efficient and compact compared to the header type. Additionally, these steam coils are usually installed horizontally within the air conditioning unit, as this orientation allows for better heat transfer and distribution of the heated air. The other options are incorrect because steam heating coils are not typically installed vertically, and the header type configuration is less common for air conditioning applications.