Fill in the blanks in the letter series “l_n_o_m_lm_o” by choosing the set of letters that completes a clear and consistent pattern.

Introduction / Context:
This problem asks you to fill in several blanks in a letter series using one of the given sets of letters. The aim is to restore a structured pattern in the final string. Such questions often hide a repeated block or a symmetric arrangement, and you need to discover it by mentally or visually grouping the letters.

Given Data / Assumptions:

• Base pattern with blanks: l_n_o_m_lm_o • Candidate fillings: mnoln, mlnmn, monln, mnmno. • Each candidate letters are placed sequentially into the blanks in the order they appear. • The completed series should form a simple, meaningful letter pattern.

Concept / Approach:
When several blanks exist, trying each option by substitution is a practical strategy. After filling the blanks, you examine the resulting full string for any regular repetition or symmetry. A strong candidate will yield a recognizable repeated block, such as a sequence of consecutive letters forwards and backwards. Weak candidates usually produce a jumbled sequence without a clear rule.

Step-by-Step Solution:
Step 1: Substitute each option into the blanks. For option (c) “monln”, the series becomes: lmnoonmllmno. Step 2: Group the resulting string into equal parts. Write lmnoonmllmno as: lmno | onml | lmno. Step 3: Recognize the pattern in these groups. The first group “lmno” is letters L, M, N, O in ascending order. The second group “onml” is the same letters in exact reverse order. The third group again is “lmno” in ascending order. Step 4: This structure suggests a neat mirror pattern: lmno, onml, lmno. Such a symmetric forward and backward repetition is a strong and intentional pattern.

Verification / Alternative check:
Check the other options by substitution. None of them results in such a clean three block symmetry. They create irregular sequences that do not exhibit a simple repeated forward and backward order. Because competitive exam questions are designed with crisp logical patterns, the option that creates lmno | onml | lmno is almost certainly intended as the correct completion.

Why Other Options Are Wrong:
• “mnoln” produces a sequence without a clear division into repeated forward and reverse segments. • “mlnmn” leads to an inconsistent mixture of letters that fails to show any obvious repetition or symmetry. • “mnmno” similarly yields a jumbled result where no simple conceptual rule can be identified.

Common Pitfalls:
A frequent mistake is to focus only on local effects near each blank instead of examining the full final string. Another pitfall is to accept the first option that looks somewhat regular, rather than systematically checking all and searching for the clearest pattern. For letter series, exam setters usually design neat repeated structures rather than messy partial patterns.

Final Answer:
The correct set of letters to fill the blanks is monln.