" The scheduler re-evaluates the process priorities every T time units and decides the next process to schedule. "
~ This means there must be a time quantum T exists , without time quantum scheduler can not re evaluates the process priorities .
So now option A , FCFS gone wrong, as in FCFS there is no time quantum.
And option C , SJF also gone wrong as it is also not pre emptive scheduling algorithm.
so we left with option B and option D .
Now option D is wrong because it says " assigns priority proportional to the waiting time of a process. " .
" A scheduling algorithm assigns priority proportional to the waiting time of a process. "
~ This makes Option C and D wrong , because in SJF ( shortest JOB First ) and SRTF( it is just the preemptive version of SJF, those who have less BURST time after first process executed, executed before large burst time process ) , priority is assigned based on Burst Time of a process.
FCFS and Round Robin scheduling , priority is assigned based on waiting time of process. ( when arrival time is not same )
so we are left with only option B , which is RR scheduling .
That's why only option B is correct.