Which statement best describes a constant speed propeller's operating principle?

A constant speed propeller operates under a principle where the propeller blade pitch can be adjusted in response to changes in engine performance, specifically to maintain a desired engine RPM. This adjustment allows the propeller to optimize performance across different phases of flight, providing efficient thrust while ensuring the engine operates within its ideal power range.

When the pilot uses the propeller control, it modulates the pitch of the blades to either decrease or increase drag. By varying the pitch, the propeller can maintain a constant engine RPM against varying flight conditions like airspeed and power settings. This ability to automatically adjust blade pitch not only enhances efficiency but also sustains performance during different phases of flight, such as takeoff, cruising, and landing.

In contrast, the other statements do not accurately depict the core operating principle of a constant speed propeller. While airspeed does have some influence, it's not the primary driver for pitch adjustment. The notion of fixed thrust while varying speed oversimplifies the interaction between thrust and engine parameters, and manually adjusting propeller pitch does not apply to constant speed designs, which are designed to adjust automatically.