I think it would have to be B due to the laws of relative motion. It was easier for me to think about when I considered the opposite case. Imagine someone is on a vehicle, for example a boat, with one portal on the boat and the other on the shore. You wouldn’t expect someone casually walking through the portal on the boat while it’s moving to suddenly experience the momentum of the boat after exiting. This is because the person and the portal are both experiencing the same motion.

Going back to the original scenario, the relative motion of the people entering the portal is as fast as the trolly is moving. If there was no portal, they would experience the same force if the trolly ran into them as they would if they ran into a stationary trolly at the same speed.

You have to do funny things with the conservation of momentum when dealing with hypothetical portals because the portals allow you to abruptly change the frame of reference.

Well in the portal games I don’t think portals have ever moved

But the way I see it the “distance” you travel through the portal is the same. You don’t go through a tunnel it’s instantaneous. But one portal is moving so I think just and endpoint is moving

I’m thinking of it like a normal door

If a run through a door I will inherit my momentum like in the portal games

But if a door is quickly coming at me and I’m standing still the frame just goes around me I’m still

Momentum is transferred from the trolley to the victims. In order for energy to be conserved, when they accelerate, the trolley will slow down. Now that you think about it it’s not that different from being hit by a trolley that has no portal on it

Momentum is relative. Whether the victims move to the trolley or the trolley to the victims doesn’t make any difference.