It should be FCFS. Although I'[m ASSUMING that - when question says priority is changing, it implies the change is positive - i.e. priority is increasing.
Consider that a process is running and its priority is increasing at rate of Y . Now another process arrives and waits in a queue at time t0.
At time t1, priority of process which is running would be (t1-t0) * Y while the one which is waiting would be (t1-t0) * X.
Since Y > X, running process will always have a higher priority. SO the process in queue continues to wait .
So the best answer would be FCFS.