What must pilots do when receiving instructions over HF communications?

Answer: B — Read back all instructions

Pilots must read back all instructions when receiving them over High Frequency (HF) communications because this ensures clarity and accuracy in communication between the pilot and air traffic control. HF radio transmissions can sometimes be subject to interference, static, or other disruptions, which may lead to misinterpretation of critical instructions. By reading back the received instructions, pilots provide a confirmation that they have understood the message correctly. This read-back also allows air traffic controllers to verify whether the instructions were communicated as intended, which is crucial for maintaining safety in air traffic operations. The need for clear communication is reinforced by the potential for miscommunication in HF environments, as opposed to more stable communication methods like VHF radio, where signals can be clearer. Consequently, the practice of read-back serves as a safety protocol to mitigate any misunderstandings that may arise from the limitations of HF communications.