What must pilots obtain before entering controlled airspace?

Answer: B — Clearance from ATC

Before entering controlled airspace, pilots are required to obtain clearance from Air Traffic Control (ATC). Controlled airspace is specifically designated and is regulated by ATC to ensure the safe and orderly flow of air traffic. This clearance includes essential information such as the aircraft's intended altitude, route, and squawk code for the transponder, which helps ATC track the aircraft effectively. Clearing through ATC is critical because it allows air traffic controllers to manage traffic efficiently and prevent potential conflicts between aircraft. This interaction ensures that pilots are aware of any other aircraft in the vicinity and any restrictions that may apply while they are operating in that controlled environment. The other options, while important in various contexts, do not replace the necessity of obtaining ATC clearance prior to entering controlled airspace. Pilots do not need permission from other pilots, flight approval from their airline is typically an internal process, and weather conditions are separately monitored and managed but do not substitute the requirement for ATC clearance.