A. Thick Airfoil
B. NACA Airfoils
C. Negatively Cambered Airfoil
D. Thin Airfoil
I got this question during a job interview.
I need to ask this question from The Symmetric Airfoil in portion Incompressible Flow over Airfoils of Aerodynamics
Easiest explanation: Thin airfoils can be simulated as a vortex sheet kept along the camber line. Thick airfoils cannot be represented like this. Negatively cambered airfoils may or may not be thin and in that case, this representation might be incorrect. NACA airfoils have all types of airfoils, not thin airfoils only.