The correct option is C 17th
Since Kapil and Nikunj interchange places, so Nikunj's new position (21st from left) is the same as Kapil's earlier positon (8th from right).
So, number of boys in the row =(20+1+7)=28.
Now, Kapiil's new position is the same as Nikunj's earlier position i.e. 12th from left.
∴ Kapil's position from the right =(28−11)=17th.