That'd be gravity. But the difference is that we are not moving (at least not relative to the ground) before entering the portal, so gravity just keeps us at the bottom.

After entering the portal, we are having a movement to the speed of the entering portal as we already established. So, gravity has yet to work against this motion (which will, of course, eventually keep us on ground again because gravity does its thing. But temporarily, and depending on the actual speed, we might have a bit of air time in this scenario).

I have my doubts, that this will convince you now. However, I gotta say, it was a lot of fun discussing this completely speculative topic with you. Also, I give you, that - while I'm still 100% sure that B is the only sensible solution for this question - you helped me better understand why people might think A is correct.

Have a nice one :)