Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.

PQRSO_TT_UV_MW_V

The correct option is SNUX.

Let us analyze the given letter series:

P Q R S O _ T T _ U V _ M W _ V

The series consists of 16 letter positions in total. We can divide the sequence into 4 equal segments of 4 letters each, or observe the transformation of letters at corresponding positions across consecutive 4-letter groups.

Let us break the sequence into 4 blocks of 4 letters each:

1st group: P Q R S
2nd group: O S T T
3rd group: N U V U
4th group: M W X V

Let us verify the pattern position by position across the 4 groups:

1. 1st letter of each group: P → O → N → M (decreases by 1 position in alphabetical order: 16 → 15 → 14 → 13).
2. 2nd letter of each group: Q → S → U → W (increases by 2 positions in alphabetical order: 17 → 19 → 21 → 23). Therefore, the 1st blank (2nd position of 2nd group) is S.
3. 3rd letter of each group: R → T → V → X (increases by 2 positions in alphabetical order: 18 → 20 → 22 → 24). Thus, the 1st letter of the 3rd group is N, and the 3rd blank (3rd position of 4th group) is X.
4. 4th letter of each group: S → T → U → V (increases by 1 position in alphabetical order: 19 → 20 → 21 → 22). Therefore, the 2nd blank (4th position of 3rd group) is U.

Sequentially substituting the missing letters into the blanks gives:

1st blank: S
2nd blank: N
3rd blank: U
4th blank: X

Thus, the combination of letters that completes the series is SNUX.