Which requirement is mandatory for operating in Class C airspace?

To operate in Class C airspace, one of the mandatory requirements is to establish two-way radio communication with the air traffic control (ATC) facility providing service to that airspace. This communication is crucial because it ensures that pilots are properly informed of traffic and other pertinent information while operating in busy airspaces where ATC is actively managing aircraft movements.

When entering Class C airspace, pilots must contact the ATC and be acknowledged before entering. This requirement supports safety and helps in the efficient management of aircraft in areas with higher traffic density. It is a fundamental component of maintaining situational awareness and ensuring compliance with ATC instructions.

Other options may relate to operational procedures or restrictions but do not pertain to the strict requirements of entering and operating within Class C airspace.