You almost arrived at the correct answer. The problem is that would happen. More specifically we know that four fundamental interactions can pass through the portals, e.g. a thing that is whole before passing through portals is still whole after passing through them, though for any pair of particles, atoms, neutrons and protons etc. there was a moment in time where they were separated by the portal. If you imagine the portal moving towards the pole at some speed and in the middle of it suddenly stopping, the momentum of the part on the other side would slightly pull on it, and the pole would still be moving at a reduced speed. The momentum of the second part cannot disappear and will pull on the first part. The only other reasonable option is that the pole gets split, but that obviously is not the case. So B it is