An irregular interleaving of two series is presented: Series 1 (positions 1, 4, 7, 10...): A, B, C... D is at position 10. Series 2 (positions 2, 3, 5, 6, 8, 9...): Z(26), X(24), V(22), T(20), R(18)... the pattern is -2. P(16) is at position 9. The missing elements are P (at 9) and D (at 10). This corresponds to option (3).