For finding the skin- friction drag we need to only measure shear-stress at the top or bottom surface.
(a) Always true
(b) Always false
(c) True for flat plate, which is a symmetric airfoil
(d) Depends on the Reynolds number
This question was addressed to me in an internship interview.
Origin of the question is Laminar Flow topic in chapter Incompressible Flow over Airfoils of Aerodynamics