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If you do it in the Portal 2 engine (Portal 1 doesn't support moving portals) it seems to be A but with a slight push (google for videos).
The point is, I don't think the topology of the portals even allows for things like "conservation of momentum" to make sense (anyone can correct me here).
That's what I immediately thought, but on further thought I think it might, if you assume portals move things through some kind of force rather than by folding space itself, though something yadda yadda Dirac delta function yadda yadda, but we can assume the portals are very much heavier than the object so... Well, I'd have to work it out.