The series in which the letters skipped between adjacent letters decrease in order is (D) SYDHK.
Explanation:
Between S and Y, there are 19 letters skipped (from S to D).
Between Y and D, there are 12 letters skipped (from D to H).
Between D and H, there are 6 letters skipped (from H to K).
Between H and K, there are 3 letters skipped (from K to S).
Why other options are incorrect:
(A) AGMRV: The number of letters skipped between adjacent letters increases (2, 5, 7).
(B) HNSWA: The number of letters skipped does not decrease consistently (1, 5, 12).
(C) NSXCH: The number of letters skipped does not decrease consistently (11, 15, 9).