What is true regarding the airplane's movement in illustration 4 if the present heading is maintained?

The correct choice highlights that if the airplane maintains its present heading and intersects the 180 radial, it does so at a 75° angle. This indicates the direction in which the aircraft is moving relative to the radial, showing that the heading is essentially approaching the radial from the southwest orientation towards the northeast, suggesting a significant angle of intersection.

Understanding radials is fundamental in navigation, particularly under instrument flight rules (IFR). Radials are lines that extend outward from a navigational aid, such as a VOR (VHF Omnidirectional Range). When an aircraft crosses a radial, the angle at which it crosses indicates how directly the aircraft is approaching or moving away from the radial.

In this scenario, if the airplane's heading is properly aligned with the 180 radial, but the heading itself is at a slight angle, it results in the plane crossing it at an angle of 75°. This gives insight into how an airplane's path is oriented relative to navigational aids and aids pilots in understanding their position and desired flight paths accurately.