The _________ on the steam turbine functions as a limit switch.

The overspeed trip on the steam turbine is critical for safety and operational integrity. It serves as a limit switch designed to protect the turbine from operating at dangerously high speeds. When the speed of the turbine exceeds its predetermined safety threshold, the overspeed trip mechanism activates, triggering an emergency shutdown or a trip of the turbine system. This prevents potential damage to the turbine and associated equipment, which could lead to catastrophic failure or safety hazards.

In contrast, the other options do not function specifically as limit switches. Speed control typically adjusts the turbine's operational speed without acting as a fail-safe mechanism. A pressure gauge monitors the pressure within the system but does not actively intervene in the operation to limit speed. Similarly, a temperature regulator controls the temperature but does not function as a protective limit switch for speed. Thus, the unique role of the overspeed trip makes it the appropriate choice in this context.