Series

The pattern is: Term, Reverse(Term), Term+NextLetter, Reverse(Term+NextLet ter), ... 1: AB 2: BA (Reverse of 1) 3: ABC (AB + C) 4: CBA (Reverse of 3) 5: ABCD (ABC + D) 6: Should be Reverse of 5 (ABCD) ⇒ DCBA. This matches option (3).

Analyze position-wise: 1st letters: O, P, Q, R, ... Next is S. 2nd letters: T, U, V, W, ... Next is X. 3rd letters: E, F, G, H, ... Next is I. Combining them gives SXI. This matches option (3).

Two interleaved series, but placement is irregular: Series 1 (Positions 1, 4, 7, 10...): A, B, C, ... Next is D (at position 10). Series 2 (Positions 2, 3, 5, 6, 8, 9...): Z(26), X(24), V(22), T(20), R(18), ... Pattern is-2. Next is P(16) (at position 9). The missing terms are P (at 9th) and D (at 10th). This matches option (3).