Final answer:
In a Piper Seminole's gear system, the hydraulic pump is turned off by a pressure switch when it reaches approximately 800 PSI during gear extension.
Step-by-step explanation:
In the Piper Seminole aircraft, the gear extension system operates in a way that is common to many retractable gear aircraft. When the gear is extended, the pressure switch in the hydraulic system is what shuts the pump off. This pressure switch is calibrated to a specific pressure, typically around 800 PSI, at which point it deactivates the pump to avoid overpressure in the system.
Thus, the correct answer to the question is: (a) The pressure switch reaching 800 PSI shuts the pump OFF during gear extension.
It is not the closing of all three down limit switches, the gear warn unsafe light being extinguished, or the left main gear squat switch that controls the pump during gear extension, although these components play other roles in the system's safety and operation.