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It's A, folks.
The Box must maintain momentum both before and after going through the portal.
The Portal is the only object with momentum, and the Portal stops upon colliding with the pedestal.
Keep in mind, the Box's momentum is relative to the Portal, as those are the two objects we're comparing. As long as the Portal is not moving, then the Box is not moving. That's how relativity works.
Furthermore, if you treat the Orange Portal and the Blue Portal as separate objects, then you're doing it wrong. They are entangled, and they are the same object in different positions. Learn what this means, because this is important to understanding exactly why it is that portals conserve momentum to begin with.
I think that settles this thread.
OP can you delete this now?