Fill in the blanks in the letter series “MN_O_NN_M_NO” by choosing the set of letters that completes a simple repeated pattern.

Introduction / Context:
This question involves a partially hidden letter pattern. Several blanks must be filled with a specific sequence of letters, chosen from the options, so that the entire string forms a clear and regular pattern. Often such questions are built around repeating blocks of letters.

Given Data / Assumptions:

• Base pattern: MN_O_NN_M_NO • Candidate fillings: NMMN, NMON, MNOO, NMMM. • Letters are placed sequentially into the blanks from left to right. • We seek a neat repetition or symmetry in the final sequence.

Concept / Approach:
When a letter series has multiple blanks, substitution and inspection is an efficient strategy. After inserting each candidate, you scan the outcome for repeated segments or simple cycles. Because exam questions are usually designed to produce very clean patterns, the correct option typically yields a string that can be neatly divided into identical blocks or symmetric halves.

Step-by-Step Solution:
Step 1: Substitute option (b) “NMON” into the blanks. The resulting string is: MNNOMNNOMNNO. Step 2: Try to group this string into equal parts. Write it as: MNNO | MNNO | MNNO. Step 3: Recognize the repetition. Each block is MNNO, repeated three times in a row. A repeated block pattern like this is exactly the kind of regular structure expected in such questions. Step 4: Quickly check other options and notice that they do not produce a simple exact repetition of a block of letters.

Verification / Alternative check:
Check that all blocks are identical: M N N O | M N N O | M N N O. There are no extra or missing letters. This high degree of regularity is a strong signal that the candidate is correct. By contrast, other options produce strings where letters do not group into equal, identical blocks, leading to uneven or irregular patterns.

Why Other Options Are Wrong:
• NMMN: Substituting this sequence does not produce simple blocks that repeat in a clean cycle. • MNOO: This option introduces a double O structure that disrupts any neat repetition of MNNO or similar blocks. • NMMM: This leads to clusters of M that are not mirrored or regularly repeated, making the overall pattern messy.

Common Pitfalls:
A common error is to accept a partial pattern, such as noticing MN or NO appearing several times, without confirming that the entire string breaks into identical segments. Another pitfall is failing to actually write or imagine the completed string, which can cause small irregularities to go unnoticed. Systematically testing each option avoids these issues.

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