Final answer:
The airside area that requires ATC clearance for entry is the Controlled Taxiway. It is closely managed to maintain safety and avoid conflicts between aircraft and vehicles.
Step-by-step explanation:
The airside area that cannot be entered unless clearance has been obtained from ATC (Air Traffic Control) is a) Controlled Taxiway. ATC is responsible for the safe and orderly flow of aircraft on the ground and in the air. Controlled taxiways are part of the controlled movement area that requires specific ATC clearance for an aircraft to enter or move through. This ensures that there are no conflicts with other aircraft or vehicles and maintains safety. The apron, while close to active taxiways and runways, does not generally require clearance from ATC for aircraft to use. The Instrument Landing System (ILS) is a type of navigation system that guides aircraft during approach and landing, but the area itself is not accessed by aircraft, rather it must remain clear of obstacles. Similarly, sensitive electronic equipment areas need to be protected from interference, but typically, clearance from ATC is not needed for entry since they are often secured areas not intended for general access or aircraft operations.