What does illumination of the F/O's sidestick priority arrow indicate?

The illumination of the First Officer's sidestick priority arrow indicates that the Captain has taken control of the aircraft by pressing the sidestick takeover button. This action is designed to ensure that when the Captain needs to override the First Officer's inputs for any reason—such as an emergency or a critical situation—their commands take priority.

The priority system is crucial for maintaining safe operations and clear command hierarchy in the cockpit. When the Captain's takeover button is activated, the First Officer’s sidestick inputs are then disregarded, signifying that the Captain's commands will govern the flight controls. This functionality is essential for ensuring that there is no ambiguity about which pilot has control, especially in high-pressure situations where clarity is vital for safety.

The other options refer to different scenarios that do not involve the priority arrows directly. For example, while the implications of a failure in either of the sidesticks are significant, they do not result in the illumination of the First Officer’s priority arrow. The priority system is designed specifically to facilitate clear and effective communication between pilots while ensuring that, in moments of need, one pilot can take precedence swiftly and decisively.