In the following questions a letter-series is given, in which some letters are missing. The missing letters are given in the proper sequence as one of the alternatives. Choose the correct alternative: _ nmmn _ mmnn _ mnnm _

Introduction / Context:
This letter series involves a simple repetition of a short four letter block. Some letters are missing and must be filled so that the completed string exhibits exact repetition. The incomplete pattern is _ nmmn _ mmnn _ mnnm _. We must decide which sequence of four letters, when placed into the blanks, yields a neat repeating structure.

Given Data / Assumptions:
- Incomplete series: _ n m m n _ m m n n _ m n n m _.- There are four blanks in total.- Each option gives a four letter sequence to be inserted from left to right.- The completed series is expected to consist of identical four letter blocks repeated several times.

Concept / Approach:
We test each option by inserting its four letters into the blanks and then examine whether the resulting letter string can be broken into equal blocks of four letters, all of which are exactly the same. This is a typical pattern in such alphabet series questions. The option that produces a clean repetition is the correct answer.

Step-by-Step Solution:
- Represent the pattern with underscores: _ n m m n _ m m n n _ m n n m _.- Insert option d: nnmm into the blanks in order.- The completed series is: n n m m n n m m n n m m n n m m.- Group this string into four letter blocks: (n n m m), (n n m m), (n n m m), (n n m m).- All blocks are identical, so the pattern is a perfect repetition of nnmm.

Verification / Alternative check:
- Try option a: mnnm; after substitution, the resulting groups of four letters are not all the same.- Similar inconsistency appears with options b, c and e; each leaves us with at least one block that differs from the others.- Only option d produces four identical segments, confirming it as the valid choice.

Why Other Options Are Wrong:
- mnnm, nmmn, nmnm and mnmn all yield mixed blocks after substitution, so the final series cannot be described as a repetition of a single fixed four letter pattern.- This contradicts the typical design of such letter series questions where a neat repeating word underlies the entire string.

Common Pitfalls:
- Not checking the entire completed string and judging only by local matches can lead to wrong choices.- It is easy to confuse similar looking sequences like nnmm and nmmn when scanning quickly.- Failing to group letters into fixed size blocks hides the underlying repetition.

Final Answer:
Filling the blanks with nnmm produces four identical blocks nnmm, nnmm, nnmm, nnmm, so the correct sequence is nnmm.