What characterizes a fly-by-wire system?

A fly-by-wire system is defined by its use of electronic signals to control the aircraft's flight surfaces instead of relying on traditional mechanical linkages. This modernization allows for more precise control and enhances the aircraft's responsiveness to pilot inputs. The system translates the pilot's commands into electrical signals, which then command actuators to move the control surfaces, providing benefits such as weight reduction, increased system reliability, and the ability to implement advanced flight control laws that can improve stability and performance.

In contrast, the other options involve traditional control methods or do not reflect the primary characteristic of fly-by-wire systems. Systems that use mechanical controls or hydraulic systems do not align with the fundamental principle of electronically managed control surfaces. While manual backup systems may exist, they are not intrinsic to the definition of fly-by-wire, which primarily focuses on the electronic signal replacement of mechanical controls.