In the following letter series x_zz_xx_z_yx, which set of four letters, when placed sequentially in the blanks, will complete the series to form a regular repeating pattern?

Introduction / Context:
This question deals with completion of a letter series by inserting a specific set of letters into blanks. The given string is x_zz_xx_z_yx, and we must decide which four letter sequence completes it so that the overall pattern becomes regular. Such questions test the ability to detect hidden repetition and symmetry, which are important skills in logical reasoning and coding based series problems commonly found in aptitude exams.

Given Data / Assumptions:

• Base pattern: x_zz_xx_z_yx.
• There are four blanks that must be filled in order.
• Answer options for the four letter sequence are: xyxy, xxxy, yyyz, zyyy.
• Each option supplies exactly four letters that must be inserted sequentially into the blanks.
• We look for a simple and consistent pattern, often repetition or symmetry.

Concept / Approach:
The concept is to treat the series as a whole word and evaluate each option by actually filling in the blanks. After inserting a candidate sequence, we check whether the resulting full string demonstrates a clear and systematic pattern, such as repeated blocks of letters. The correct answer should produce a neat and regular arrangement rather than a random looking one. Symmetry, repetition, and balanced structure are strong indicators of a correct completion in such questions.

Step-by-Step Solution:
Step 1: Identify the blank positions in x_zz_xx_z_yx. There are four blanks, and we will denote the final string after insertion for each option.Step 2: Insert option a) xyxy to get xxzzyxxxzyyx. This string does not show any clear simple repeating block and looks irregular.Step 3: Insert option b) xxxy to get xxzzxxxxzyyx. Again, the arrangement is uneven and does not form a clean repeated pattern.Step 4: Insert option c) yyyz to get xyzzyxxyzzyx. This can be seen as xyzzyx repeated twice: xyzzyx xyzzyx, which is a very clean and regular repetition.Step 5: Insert option d) zyyy to get xzzzyxxyzyyx, which does not exhibit a symmetric or easily recognisable repeating structure.Step 6: The only option that converts the series into a clearly repeated block is option c, giving xyzzyxxyzzyx.

Verification / Alternative check:
Observe the string formed using option c) yyyz once more. The first six letters are xyzzyx and the next six letters are again xyzzyx. This exact repetition confirms a deliberate and systematic pattern which is typical of exam series. No other option produces such a neat repetition. Therefore, option c is consistent with the logic of the series and passes a strong pattern check.

Why Other Options Are Wrong:
Options a, b, and d create irregular arrangements where no simple unit repeats across the series. They do not form symmetric structures or obvious cycles, so there is no strong logical reason to prefer them. As a result, they are considered incorrect for this series completion problem.

Common Pitfalls:
One common mistake is to rely on intuition without actually writing out the full string for each option. Another error is to focus on only part of the sequence and miss the full repeated block. For such questions, it is safer to expand all candidates and visually check for obvious repetition or symmetry before deciding on the answer.

Final Answer:
The correct set of letters to complete the series is yyyz.