Which of the following increases drag on an aircraft?

Increasing surface roughness on an aircraft is correct as it contributes to increased drag. When the surface of the aircraft becomes rough, it disrupts the smooth flow of air over the wings and fuselage, leading to a higher level of friction and turbulence. This turbulence increases the overall drag experienced by the aircraft during flight.

In aerodynamics, a smooth surface facilitates laminar flow, which minimizes drag, while a rough surface engenders more drag due to increased frictional resistance and energy loss in the boundary layer of airflow. This principle is particularly significant in aircraft design and maintenance, where keeping surfaces clean and smooth is paramount for optimizing performance and fuel efficiency.