The lift curve slope of a two-dimensional airfoil in incompressible flow is typically described by the thin airfoil theory, where the slope is constant at approximately \(2\pi\) rad/degree per unit change in angle of attack. However, as the Mach number increases and approaches compressibility effects (and especially if considerably higher than 0.3), we begin to see changes due to compressibility.
At higher Mach numbers, compressibility effects alter the aerodynamic characteristics of the airfoil, and the lift curve slope generally decreases. This is primarily due to the onset of shock waves and changes in flow behavior around the airfoil, which affect the effective lift generation.
Therefore, the correct answer is:
Decreases.