Actually, the portal video demonstrating this thought experiment that was posted earlier is probably the most accurate. The blue portal and the block are never accelerated relative to eachother. The block would never penetrate the blue portal. So the choice between A and B is false. If we assume that the block could emerge from the blue portal while the blue portal remained stationary, then the block would have to accelerate relative to the blue portal. Whether one wants to think of the block as being pushed out or pulled out in that hypothetical scenario is a matter of taste. Conservation of momentum (the "memory" of the block's movement) would be out the window. "We can't stop here: this is paradox country."