Final answer:
High-lift devices like flaps and slats are used during takeoff and landing to enhance lift at lower speeds and aid in deceleration and approach angle during landing.
Step-by-step explanation:
High-lift devices, such as flaps and slats, are crucial components in aircraft design used to increase the wing's lift during specific phases of flight. The correct answer to the question about when high-lift devices should be used is c. during takeoff and landing. During takeoff, these devices allow the aircraft to generate the necessary lift at slower speeds, while during landing, they help to slow the aircraft down and enable a steeper approach without gaining speed. High-lift devices are not required when breaking the sound barrier.