Select the option that is related to the third group of letters in the same way as the second group of letters is related to the first group of letters:

SKDG : OOZK : : RDPC : ?

NHLG

    S      K     D       G                   R        D     P      C

-4 ↓ +4 ↓  -4 ↓  +4 ↓                 -4 ↓  +4 ↓  -4 ↓  +4 ↓

   O     O      Z       K                    N       H      L      G

So correct answer is a.