How is the hydraulic system kept functional during operation?

The hydraulic system's functionality during operation is primarily maintained by preventing foaming within the hydraulic fluid, which is why the correct choice is focused on the use of engine bleed air. Foaming can severely compromise the performance of a hydraulic system by introducing air into the hydraulic fluid, which reduces its effectiveness and can lead to a loss of pressure and responsiveness in the system. By utilizing engine bleed air to keep the hydraulic fluid in an optimal state, the potential for foaming is minimized.

This practice allows for consistent and reliable operation of hydraulic systems, ensuring they can perform the necessary functions, such as controlling flight surfaces or other critical operational systems. The other options, while they may contribute to operational performance in various contexts, do not directly address the challenge of maintaining hydraulic fluid integrity and preventing foaming in the way that engine bleed air does.