B, even as the people do not have momentum, the portal does. Think of it this way, the first person coming out of the portal, let's say they are having zero speed, but he is right infront of the portal, the second person to exit will exert a force on the first person to move and create space so that the second person can be next to the portal. What speed is this second person coming through the portal, why that will be similar speed to the moving portal. Only if the second portal is moving away at a similar speed will the people experience a situation like A, but as can be seen portal A is fixed to an angled block