In the following letter series with gaps, M__OMMN__M__NOMMN__M, which one set of letters, when placed sequentially in the four gaps, will correctly complete the pattern?

Introduction / Context:
This problem is a letter series completion question where a repeated pattern of letters involving M, N, and O is partially hidden by gaps. The given series is written as M__OMMN__M__NOMMN__M, and we are asked to choose a specific sequence of four letters that, when filled into the gaps in order, makes the whole string follow a consistent pattern. Such questions test pattern recognition in repeated sequences and attention to the order in which missing letters are filled.

Given Data / Assumptions:

• Given series with four gaps: M__OMMN__M__NOMMN__M.
• The same four letters are placed in the gaps from left to right in the order given by the option chosen.
• Options are sequences of four letters: "O N M O", "N O M O", "M O N M", "N N M O", and "M N O N".
• We assume that the final completed series should display a regular repeated pattern in terms of the arrangement of M, N, and O.

Concept / Approach:
The key idea is to notice that the visible parts of the series already hint at a repeating cluster of letters. The fragments "OMMN" and "NOMMN" appear, suggesting that some form of the block "NOMMNO" or "MMNOOM" repeats. To determine which set of letters produces a fully regular pattern, we must fill each gap with the letters of a chosen option in the given order and then examine the resulting string to see whether a repeating block emerges without contradictions or random breaks.

Step-by-Step Solution:
Step 1: Label the four gaps in order from left to right as gap 1, gap 2, gap 3, and gap 4 in the series M__OMMN__M__NOMMN__M. Step 2: Try option B, which provides the sequence N, O, M, O for the four gaps. Insert N in the first gap, O in the second, M in the third, and O in the fourth. Step 3: After filling with option B, the series forms a long string where groups like NOMMNO and MMNOOM appear more than once, indicating a uniform repeating structure. Step 4: Now compare this behaviour to what happens if you try other options. When options such as A, C, D, or E are inserted, the resulting string does not display a clean repeated segment; you may see sudden breaks where the pattern does not align smoothly. Step 5: The goal in such a question is to maintain a consistent pattern of M, N, and O blocks throughout the sequence. The combination N O M O creates symmetrical repeats around the central "MMN" and "NOMM" fragments that are already present.

Verification / Alternative check:
A quick way to verify is to focus on the central visible fragment "NOMMN" and check how the filled letters join this fragment on both sides. With option B, the letters before and after this fragment can be grouped to give the same arrangement of M, N, and O repeated multiple times, while the other options either force double Ns or disrupt the sequence in a way that cannot be grouped into a repeated pattern. This makes option B the only choice that preserves consistency across the entire series.

Why Other Options Are Wrong:
Option A "O N M O" leads to awkward triples of identical letters in some places and breaks the repeating structure around "OMMN".
Option C "M O N M" creates mismatched groups that do not align with the existing visible parts "OMMN" and "NOMMN".
Option D "N N M O" introduces a double N sequence at places where the pattern requires a transition through O or M to maintain symmetry.
Option E "M N O N" similarly produces unmatched clusters that do not repeat cleanly and conflict with the visible fragments.

Common Pitfalls:
A typical mistake is to fill the blanks with one option and accept it without scanning the entire resulting series. It is important to examine how the filled letters interact with existing letters throughout the string, not just near one gap. Another pitfall is to look for overly complex numeric rules instead of recognizing that this question is about visual repetition of short blocks like NOMMNO. Carefully checking whether the final series looks like a smooth repetition of a single pattern helps avoid these errors.

Final Answer:
The set of letters that correctly completes the series is N O M O (option B).