Thanks to all who answered
Well, so far as I can tell, you have unanimously chosen B , so I guess B is the safest approach
It's not clear to me from your answers whether (at the next scrum) you make the replacement leave the pitch, or whether red get to choose any player they like to leave?
For me I think that when man off came in, A would have been the intended answer. (It was always emphasized to me that it's not about making someone leave the pitch, it's that the player leaving the pitch cannot be replaced )
But B is certainly closer to the approach used when YC a front row , so at least that makes sense.