Which one set of letters, when sequentially placed in the blanks of the following letter series, will correctly complete it: ___ utt ___ tt ___ ttu ___ ?

Introduction / Context:
Letter series completion questions test a learner's ability to recognise repeating patterns and fill in missing segments. In this problem, a short sequence of letters involving t and u is given with four blanks. We must choose the one set of letters that, when inserted in order, produces a consistent and repeating pattern. Such questions reward careful observation and the ability to rewrite the sequence without getting confused by the blanks.

Given Data / Assumptions:
- Original incomplete series: ___ utt ___ tt ___ ttu ___
- There are four blanks, all to be filled by a single set of four letters in a fixed order.
- All letters used in the options are t and u only.
- The completed series should show a clear, regular repetition of some smaller pattern.

Concept / Approach:
A good method is to represent the full sequence with symbolic placeholders, insert each option in turn, and then see whether the final pattern is consistent. Because the visible fragments include "utt" multiple times, it is natural to suspect that the underlying full series might be a repeated block like "tutt" occurring several times. We therefore test each option to identify which one generates this neat repetition.

Step-by-Step Solution:
Step 1: Rewrite the incomplete series as a continuous string with blanks: _ u t t _ t t _ t t u _. Step 2: For option A (t t u t), substitute into the blanks to get: t u t t t t t u t t u t. This does not form a simple repeating pattern. Step 3: For option B (t u t t), substitution gives: t u t t u t t t t t u t. This again does not show a clear, repeated block structure. Step 4: For option C (t u u t), substitution yields: t u t t u t t u t t u t. Step 5: Grouping this result gives: tutt tutt tutt, a repeated block "tutt" three times. This is a very clean, regular pattern. Step 6: For completeness, test option D (t t t u). It produces: t u t t t t t t t t u u, which does not produce a simple repeating structure.

Verification / Alternative check:
Take the completed series from option C: t u t t t u t t t u t t. Split it into blocks of four letters: tutt, tutt, tutt. Each block is identical, confirming that the sequence is a simple repetition of "tutt". This strong regularity makes option C the clear and unique correct choice among the given sets.

Why Other Options Are Wrong:
- Option A produces mixed clusters without a clear repeating four letter pattern.
- Option B gives an uneven arrangement where the distribution of u and t is not regular in blocks of equal size.
- Option D leads to an arrangement with long strings of t's that do not break into equal repeating segments.
Since series questions typically expect a neat, repeating micro pattern, these options do not fit the expected logical structure.

Common Pitfalls:
Many test takers try to work mentally and lose track of positions when substituting multiple blanks. It is far safer to write the entire sequence out, label positions, and then insert the option letters clearly. Another frequent error is to stop after testing only one or two options without checking fully for repetition. Systematically verifying each option prevents guesswork and improves confidence in the final choice.

Final Answer:
The set of letters that correctly completes the series is t u u t (option C).