Non-preemptive shortest job first (sjf) scheduling policy is not optimal if:

a) all the jobs arrive at the same time
b) the cpu burst run-time of each job is known
c) each job runs for the same amount of time
d) jobs may arrive at different times

The answer is A

Explanation:

Basically, Shortest job first (SJF) is a scheduling policy that selects jobs on queue for execution  within a short execution time.

From the definition of SJF above, it means that there are a lot of process on queue and the (SJF) job is to receive processes on queue to execute within a short execution time.

Therefore, if all the jobs or process arrives at the SJF at the same time, the SJF will forfeit one of its major purpose which is scheduling of jobs.

That will therefore not make Non-preemptive Shortest Job First (SJF) not function at its optimal point.