The series is having 5 multiplication table . that is 5,10,15,20,25,30. If you add positions of individual elements present A through Z series you will get the above derived positional series .
Explanation : B's position is 2 and C's position is 3 . 2+3=5 , H's position is 8 , A's position is 1 which leads to 8+1+1=10, same way O's position is 15 . Using same logic we must get the resulting letters total positions value as 30 . In the given options D and Z is suitable . Since D's position is 4 and Z's position is 26 whose some is 26+4=30 .