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Everyone that says B is stupid.
As the surface the orange portal is hitting the surface the cube is on, it comes to a stop, it cannot proceed anymore.
Now what does that mean for the blue portal? Well if you try to go through the blue one, it's just a flat, solid surface.
The block, at no point in time will have force applied on it. When the orange portal touches it, the cube is still in equilibrium. When the 2 surfaces collide, as mentioned above, both stop and there is a solid surface where the blue portal is.
The cube would (after not being acted upon by any external force) simply slide down the side of the.,..what ever the blue portal is on