What phraseology should be used to request a pilot to validate an altitude below the lowest usable flight level?

The phraseology "Say altitude" is the appropriate choice for requesting a pilot to validate an altitude below the lowest usable flight level (LUF). This phrase clearly instructs the pilot to communicate their current altitude, which is crucial for maintaining safe separation and ensuring compliance with altitude restrictions, especially when operating below designated flight levels.

Using "Say altitude" is precise and standard in air traffic communication, facilitating clarity and minimizing the possibility of misunderstanding. It directly prompts the pilot to provide the information needed at that particular moment, ensuring that air traffic controllers can assess the situation accurately.

In contrast, other options like "Check altitude" or "Confirm height" may not convey the urgency or specific requirement for the pilot to provide their current altitude. Additionally, "Report current altitude" might seem similar but does not align with the established phraseology conventions associated with situations involving safety concerns at low altitudes.