The correct answer is A) start and stop the compressor as needed. The low-pressure cut-out switch is a safety device used as a primary controller for a refrigeration system. Its primary purpose is to monitor the low-side pressure (suction pressure) of the system and automatically shut off the compressor when the pressure drops below a preset level. This prevents damage to the compressor that could occur if it were to continue running under a vacuum condition. The other answer choices are incorrect because: B) the compressor capacity is typically controlled by other devices, C) the low-pressure cut-out switch maintains a minimum suction pressure, not a preset pressure, and D) the low-pressure cut-out switch does not directly control the low-side pressure, but rather protects the system by shutting off the compressor when the pressure drops too low.