What system uses an onboard air compressor which adds air to the mix before it is discharged from the pump?

The correct answer is the system that uses a compressed air foam system (CAFS). This system works by combining water with foam concentrate and then introducing compressed air into the mixture before it is discharged from the pump. The addition of air results in a foam that is lighter and has improved expansion qualities, which enhances its effectiveness in suppressing fires. This unique air inclusion helps create a more stable and effective foam blanket that adheres to surfaces, maximizing cooling and smothering effects.

In contrast, other systems listed do not utilize an onboard air compressor in the same manner. Foam proportioning systems typically focus on mixing foam concentrate with water but do not introduce air into the mix before discharge. Dry chemical systems rely on powder-based extinguishing agents rather than foam, and water mist systems utilize fine water droplets rather than foam, focusing on cooling and displacing oxygen to suppress flames without the need for added air into the mixture in the same way that CAFS does.