Correct answer is (c) β^2ϕxx + ϕyy = 0
Easy explanation: For a compressible subsonic flow over a thin airfoil, the two dimensional linearized perturbation velocity potential equation is given by
β^2ϕxx + ϕyy = 0
In this equation the perturbations are assumed to be small with the value of β = \(\sqrt {1 – M_∞^{2}} \).