Which set of letters when sequentially placed at the gaps in the given letter…

Concept: A repeating-block letter series is solved by finding the smallest group of letters that repeats across the whole sequence, then using the letters already given in that block to fix every blank inside it.

Application:

1. Split the 12-letter series T_ST_S_U_T_S into four groups of three letters: T_S, T_S, _U_, T_S.

2. The first, second and fourth groups all show T _ S, so the repeating unit is T-U-S, with U as the fixed middle letter.

3. The first group T_S needs U in its gap to read T-U-S.

4. The second group T_S needs U in its gap to read T-U-S.

5. The third group _U_ already has U in the middle, so its two gaps need T and S to read T-U-S.

6. The fourth group T_S needs U in its gap to read T-U-S.

7. Reading the five gaps left to right in the order they occur gives U, U, T, S, U.

Cross-check: Substituting U, U, T, S, U back into the gaps reconstructs T U S / T U S / T U S / T U S — four identical T-U-S blocks — confirming the pattern holds across the whole series.

The set of letters that completes the series is U, U, T, S, U.