What causes friction drag?

Friction drag is primarily caused by the interaction of air molecules with the surface of the aircraft. As the aircraft moves through the air, the air molecules adhere to the surface, creating a resistance to the flow of air. This resistance is due to the viscosity of the air and occurs when the layers of air in contact with the skin of the aircraft are slowed down compared to the airflow further away. The boundary layer formed near the surface is where this friction occurs, and it leads to a drag force that opposes the forward motion of the aircraft.

Other factors listed in the question do not directly contribute to friction drag. For instance, the weight of the aircraft affects how the aircraft interacts with air but does not directly cause friction drag. Similarly, induced turbulence refers to the chaotic movements of air as a result of lift generation and does not inherently cause friction drag on the surface itself. Vertical motion in the air can affect overall drag, but it is not a defining factor in the friction drag experienced by an aircraft's surface. Thus, understanding that friction drag results from air resistance acting on the aircraft's surface clarifies why that particular choice is the most accurate.