Split each block into its 1st, 2nd and 3rd letters and check the jump between them. 1st letters: A(1), G(7), M(13), S(19) — each jumps by +6, so next is 25 = Y. 2nd letters: F(6), L(12), R(18), X(24) — jump by +6, next is 30, which after wrapping past Z(26) becomes 30-26 = 4 = D. 3rd letters: C(3), I(9), O(15), U(21) — jump by +6, next is 27, wrapping to 27-26 = 1 = A. So the answer is YDA, option (1).