Assuming the black platform does not fit through the orange portal, and the moving panel with the orange portal stops on impact (which is what appears to be true because on the blue portal side we see the surface of the black panel), then the cube is no longer moving relative to the orange portal and therefore has no momentum. Once the cube is entirely on the blue side of the portal it will be affected by the gravity on that side and slide down as if on a ramp (A).
Assuming the black platform does not fit through the orange portal, and the moving panel with the orange portal stops on impact (which is what appears to be true because on the blue portal side we see the surface of the black panel), then the cube is no longer moving relative to the orange portal and therefore has no momentum. Once the cube is entirely on the blue side of the portal it will be affected by the gravity on that side and slide down as if on a ramp (A).