The first letter is one place, second is two placed, third is three places, fourth is four places, fifth is five places and sixth is six places before the coressponding alphabet in the given code.
Hence, the answer is (c).
(N → O),
(E → F → G),
(O → P → Q → R),
(M → N → O → P → Q),
(A → B → C → D → E → F),
(N → O → P → Q → R → S → T).

Same as
(Y → Z),
(I → J → K),
(Z → A → B → C),
(H → I → J → K → L),
(P → Q → R → S → T → U),
(J → K → L → M → N → O → P).