Yeah but the momentum is relative to the portal.

If the blue exit portal was behind the wagon and so moving at the velocity of the orange entry portal, then I would agree that it's A because they move at the same velocity and in the same direction.

But since the blue exit portal is static and the orange one is moving, the people will enter the portal at a relative velocity to the portal which will be transferred to the blue one. Meaning B will occur.

If the portals were on two wagons going in the opposite directions at the same X velocity, then the people would enter at X relative velocity and exit at 2X velocity.