When does the [HYDR FL LO] annunciator illuminate?

The annunciator [HYDR FL LO] is designed to alert the crew about low hydraulic fluid levels within the system. It specifically illuminates when the reservoir level drops below a critical threshold, in this case, 1 quart. This is vital because maintaining an adequate fluid level is crucial for the hydraulic systems to function effectively; insufficient fluid can lead to decreased performance, potential system failures, and safety hazards.

Understanding the importance of hydraulic fluid levels is essential in ensuring that the aircraft operates within safe parameters. Other options relate to different parameters such as pressure, fluid loss rate, or temperature, which do not specifically pertain to the fluid level indicator. Therefore, the illuminating condition of the [HYDR FL LO] annunciator unequivocally correlates with the reservoir level falling below 1 quart, which is why this is the correct answer.