I think in this scenario it's pretty clearly B. Though I have a secondary scenario. What if you're standing against a wall and the portal slams into that wall. You're against the wall but the wall itself never goes through the portal. Would you still get launched then? The portal is still moving when you go through it but stops abruptly as soon as it reaches the wall. You're still physically touching the wall that's now on the other side of the portal. Would you get forced off of it and launched?