In this series odd placed letters are random letters but even placed letters are having relation with previous odd positioned letters . That means H is related to Z , F is related to O so on. Now let us identify the relation using A through Z series of letters positions .Z's position is 26, After adding individual numbers of Z position leads to 8 and 8th positioned letter H . Same way O's position is 15 , if you add every number present in this position leads to 6 . 6th position letter is F . using same logic you will get G as answer .
Letters Position reference :
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26