Think of a portal like a doorway or just a frame of one. Stepping through it seamlessly puts you on the other side. If you were standing still, and the door went around you, would any force be exerted upon you, causing you to move? That’s why I think it would be A. Nothing is applying any force to you, you’re just basically teleporting. You may not even actually fully emerge from the portal, being trapped on the threshold, since no force is moving you beyond the point where the two ends meet.