Which category of fire apparatus includes an apparatus used primarily for initial response to incidents?

The category of fire apparatus that includes an apparatus used primarily for the initial response to incidents is indeed the pumper. Pumpers are designed to carry water and are equipped with pumps that can deliver water at high pressure, making them essential for combating fires right from the start. They often carry hoses, nozzles, and various firefighting tools, allowing firefighters to begin suppression efforts immediately upon arrival at an incident scene.

In contrast, mobile water supply apparatus are primarily used for transporting large quantities of water, especially in areas where a hydrant system may not be available. Special service apparatus is used for specialized operations such as hazardous materials response, technical rescue, or EMS support, rather than standard fire suppression. Aerial apparatus, while important for reaching high-rise fires or performing rescues, are not primarily designed for initial suppression and typically arrive after the initial response has begun. Thus, the pumper's role as the first responder to incidents makes it the correct answer in this context.