Correct choice is (b) False
For explanation I would say: According to the Prandtl – Glauert rule, the incompressible pressure coefficient is given by:
Cp = \(\frac {C_{p_0}}{\sqrt {1 – M_∞^{2}}}\)
For airfoils which are thin, the flow has less expansion resulting in less magnitude of Cp0. This results in a higher critical Mach number value. On the other hand thick airfoils have higher magnitude of Cp0 because of strong flow expansion. This results in lower critical Mach number.