In the letter series L M _ L _ N L _ N _ M _, which one set of letters, when placed sequentially in the blanks, will correctly complete the pattern?

Introduction / Context:
This is another letter series completion question, this time involving the letters L, M, and N. Several positions are missing and must be filled by choosing one of the options. The aim is to reconstruct a neat repeating pattern. Typically such series are arranged so that once you find the short building block, the entire sequence becomes very simple and symmetric.

Given Data / Assumptions:

• Incomplete series: L M _ L _ N L _ N _ M _.
• There are five blanks; we must insert five letters from a single option in the given order.
• We expect the complete series to be made of repeated blocks using L, M, and N.

Concept / Approach:
The letters appear in the order L, M, and N at various points, suggesting that the underlying block is L M N. A natural hypothesis is that the entire sequence is a repetition of L M N several times. We confirm this by testing options and checking which one recreates the block L M N repeatedly with no break. This method uses both local alignment of letters and global repetition to identify the correct answer.

Step-by-Step Solution:
Step 1: The blanks are at positions 3, 5, 8, 10, and 12. Step 2: Choose option B, which provides the letters N M M L N. Step 3: Fill the blanks to obtain the full series: L M N L M N L M N L M N. Step 4: Group the completed sequence as LMN LMN LMN LMN. Step 5: Each group of three letters is exactly L M N, repeated four times consecutively. Step 6: There is no inconsistency in this arrangement, so option B creates a perfectly regular pattern.

Verification / Alternative check:
Verification is simple: check every three letter block. Positions 1 to 3, 4 to 6, 7 to 9, and 10 to 12 all read L M N. There are no duplicated letters where they do not belong and no missing ones in any position. Other options, when tested, give sequences that cannot be grouped into identical LMN triplets, which confirms that option B is unique in generating a complete and consistent pattern.

Why Other Options Are Wrong:
Option A (NNNLL) produces clusters of N and L that break the LMN triplet structure. Option C (LMLMN) results in an irregular mixture of letters that cannot be partitioned into equal LMN blocks. Option D (NMLMM) also fails to yield identical triplets and introduces repeated M letters where the order demands N instead. Because only option B produces the clean repetition LMN LMN LMN LMN, the other options are incorrect.

Common Pitfalls:
A typical error is to consider only immediate neighbours and decide based on whether each small transition looks plausible, such as L followed by M. However, a few local matches do not guarantee that the whole sequence follows a uniform rule. Another pitfall is to overlook the possibility that the pattern might revolve around a three letter block rather than a longer or more complicated structure. Always attempt to group the completed series into equal sized chunks and verify that each chunk is identical.

Final Answer:
The correct set of letters is option B, NMMLN, giving the series LMN LMN LMN LMN.