What expels the extinguishing agent from a pump tank water extinguisher?

In a pump tank water extinguisher, the extinguishing agent is expelled from the tank through manual effort. This involves the firefighter physically operating the pump mechanism. As the firefighter pumps the handle, internal pressure is created, pushing the water out through the nozzle. This direct manual action is essential for controlling the flow and direction of the water being discharged, allowing for effective application of the extinguishing agent on a fire.

The other options do not apply because electricity is not needed to operate this type of extinguisher, internal pressure alone from a built-in mechanism would not effectively allow precise control without manual operation, and gas compression is not a factor in the functioning of pump tank water extinguishers since they rely solely on the action of the firefighter to create pressure and expel the water.