What constitutes the landing gear of an aircraft?

The landing gear of an aircraft is specifically designed to support the aircraft during takeoff, landing, and while it's on the ground. This component typically includes the wheels and associated structures, such as the struts and actuators that help absorb the shock during landings and provide stability on the ground. The primary function of the landing gear is to ensure a safe and stable platform for the aircraft when it is not in flight.

The other options do not represent components involved in the landing phase. The wings are critical for generating lift during flight but do not serve the function of landing. The propulsion system is responsible for providing thrust to move the aircraft through the air and is not involved in ground operations. Cockpit instruments are essential for pilot navigation and aircraft operation during flight but have no role in the physical structure required for landing. Understanding the specific purpose and design of landing gear is crucial for recognizing its vital role in aviation safety.