When does a safety alert become a first priority for a controller?

A safety alert becomes a first priority for a controller when there is an observed unsafe proximity to terrain or obstacles. This is essential because the primary responsibility of air traffic controllers is to ensure the safety of all aircraft under their jurisdiction. When a controller identifies a situation where an aircraft is in danger of colliding with terrain, obstructions, or other aircraft, it necessitates immediate action.

In this scenario, the controller must prioritize the safety alert over other tasks to mitigate risk and provide appropriate instructions to the pilot. Constant vigilance regarding altitude, flight paths, and potential hazards is a vital aspect of a controller's duties, making the recognition of an unsafe situation a critical factor for intervention.

Other options suggest conditions that are either too vague or reliant on actions from other sources (such as a pilot notifying the controller or planning in advance). However, safety alerts stem from the controller’s active monitoring of the airspace and their ability to respond proactively to prevent incidents.