@Mohank it simply means that scheduling will happen if a process arrives or completes(Observe When P0 had executed for 1ms then at t=1 P1 arrives and since it is mentioned in the question that scheduling can happen if a process arrives, so srtf will now schedule process P1 because it has shorter Burst time) at T=2 again process P3 arrives then scheduling will happen again and so on.
If the question had been “Scheduling is carried out only at arrival AND completion of processes.” then it would have been non-preemptive because at t=1 even if P1 arrives the process P0 has not yet completed its execution, only at T=9 scheduling will happen because at that time a process has completed it execution and other processes have already arrived.