__________ consist of a small return (bypass) water line connected from the discharge side of the pump back to the intake side of the pump.

The correct answer, around-the-pump proportioners, plays a crucial role in mixing water with a foam concentrate for firefighting purposes. These systems require a return line that allows a portion of the water being pumped to bypass the discharge and return to the intake side of the pump. This method ensures that a consistent ratio of foaming agent is blended with the water as it flows through the pump system, resulting in effective firefighting foam application.

Around-the-pump proportioning systems operate efficiently because they maintain pressure and flow rates, allowing firefighters to effectively control the foam production during an incident. This system is especially important when working with larger fire suppression operations where foam concentration needs to be precise and reliable.

In contrast, tank-to-pump systems refer to how water is transferred from a tank to the pump, while bypass valves are mechanisms that divert water flow to prevent pump overheating or excessive pressure. Flow restrictors are used to limit the amount of water flowing through a line. While these terms are related to water flow in firefighting scenarios, they do not encompass the specific function of blending foam concentrate through a dedicated return line like around-the-pump proportioners do.