We need to find the least cpu utilization i.e. the max cpu wastage.
Consider the processes as P and Q
1. Firstly let us consider the FCFS schedule.
0 - 10 P will use the CPU usage and after that starts the I/O
11-20 Q will use CPU and after 20 ms will start the I/O
P will complete the I/O at time = 10 + 90 = 100;
Q will complete the I/O at time = 20 + 90 = 110;
Note that after 100 ms CPU is again starting to be utilized by P. Thus the first cycle ends at 100 ms.
Thus in the time of 100 sec the CPU was vacant for time = 20 to 100 as after 100 P will again use CPU for the 2nd time. This means CPU wastage of 80 ms. This gives us a CPU utilization of 20/100 = 20 percent.
2. Now let us consider the case of RR scheduling with time quantum of 5 ms.
0-5 P will use for CPU
5- 10 Q will use for CPU
10-15 P will again use for CPU and complete its required 10 ms time,
15-20 Q will use CPU and complete its required 10ms time.
From 15 onwards P will start use for I/O (please note it is 15 onwards). P will continue to use till 15+ 90 = 105.
from 20 onwards Q will start use for I/O . Q will continue to use till 20 + 90 = 110.
CPU is not used in the time 20 to 105. (NOTE that the 2nd cycle will start at the time 105ms and again the CPU becomes available to P )
ie CPU wastage = 20 to 105 = 85 ms . This gives us a CPU utilization of 20 / 105 = 19 percent
Thus the least CPU utilization in the RR case. Correct ans should be (D).
Hope this helps to clear your doubt.