Which of the following increases drag on an aircraft?

Increasing the surface area of an aircraft is a direct factor that contributes to increased drag. Drag arises from the air resistance an object encounters as it moves through it. When the surface area of the aircraft increases, more air is disturbed and pushed aside, resulting in greater resistance against the motion of the aircraft. This scenario is commonly seen in aircraft configurations such as larger wings or additional components, which create more lift and subsequently more drag.

In contrast, decreased air density normally leads to reduced drag since there are fewer air molecules to create resistance. Reduced airspeed can also result in decreased drag, as the force opposing the aircraft's motion is dependent on speed, which follows a quadratic relationship with drag: as speed decreases, drag diminishes rapidly. Altered camber can affect lift characteristics and potentially modify the drag profile of an aircraft, but this effect can vary based on many factors; generally, the primary factor for increased drag remains surface area enlargement. Thus, the option identifying increased surface area as the cause of increased drag is the most accurate.