In the series when you deduct positions of all letters in any set with the corresponding first letters position you will get 1 as result.
In case of above series z's position is 26 and y's position is 25 , 26-25 results 1 . same way R's position is 18 , M's position is 13 and D's position is 4 , which leads to 18-13-4=1 . Hence all answers are correct .