What is displayed on the lower display unit as selected on the ECAM control panel?

The correct choice is that the lower display unit shows the System Displays as selected on the ECAM control panel. The Electronic Centralized Aircraft Monitor (ECAM) system on the A320 is designed to provide a comprehensive view of the aircraft's systems. When the system displays are selected via the ECAM control panel, the lower display unit will illustrate a range of information regarding the aircraft's performance and functionality, including systems like the hydraulic system, fuel system, electrical system, and more.

Each system is represented in a way that allows for quick reference and assessment, which is essential for managing aircraft operations effectively. Pilots can cycle through various system displays to gain insight into how each component of the aircraft is performing, thus aiding in situational awareness and decision-making.