Final answer:
Dihedral is the design element that best enhances an airplane's lateral stability, promoting self-righting characteristics and better rolling resistance.
Step-by-step explanation:
When discussing the design elements that enhance a plane's lateral stability, the correct answer is d) Dihedral. Lateral stability refers to an airplane's stability around its longitudinal axis, which affects its ability to resist rolling motion. An upward angle of the airplane wings relative to the horizontal plane, known as dihedral, greatly contributes to lateral stability. When an airplane with a dihedral wing configuration is disturbed by a gust of wind causing it to roll, the lower wing experiences more effective lift than the higher wing, which creates a restoring moment and returns the plane to level flight.
In contrast, winglets help reduce induced drag by improving airflow at the wingtips, thus improving an aircraft's efficiency. A canard configuration, which is an arrangement where a small forewing is placed forward of the main wing of the aircraft, affects longitudinal stability and control rather than lateral stability. Lastly, leading-edge slats are aerodynamic surfaces on the leading edge of the wings that are used to increase the wing's lift during takeoff and landing sequences, but do not specifically improve lateral stability.