Given in system there are on an average 5 new processes arrive per minute.
So,Arrival rate = 5 process / min
In every 12 second a ( $\frac{60 }{5}$ )new process arrives in system or we can say every process stays 12 second with the CPU.
Given service time = 6 seconds
CPU Utilization =$\frac{Service \;Time}{Staying\;Time}*100 $
=$\frac{6}{12}*100$=50%
Hence,Option(B)50% is the correct choice.