ATC services are provided on which basis when situations permit?

Air Traffic Control (ATC) services operate primarily on a "First Come, First Served" basis when situations permit. This system ensures that aircraft are managed in an orderly fashion, enhancing safety and efficiency within the airspace. Under normal circumstances, it allows for the systematic handling of aircraft, where those that arrive first are processed first.

This approach serves as a fundamental principle within air traffic management, as it helps avoid congestion and potential conflicts. By following this order, ATC can manage multiple flights effectively, ensuring that all aircraft receive the necessary attention and services in a timely manner.

Other options do not accurately reflect how ATC services are typically organized. "Equal Opportunity" may suggest fairness but does not align with the structured process ATC employs. "Random Selection" would lead to unpredictability and chaos in the skies, detracting from overall flight safety. "Priority Queue," while it may sound similar, generally implies a system where some aircraft might receive prioritization based on specific criteria (like emergency status), rather than the standard practice of prioritizing based on arrival time when conditions allow.