In the incomplete series c_e_cd_f_de_, which one set of letters, when placed sequentially in the blanks, will complete the series correctly?

Introduction / Context:
This question involves completing a letter series with several blanks. The pattern c_e_cd_f_de_ must be filled using one of the provided five-letter sets so that the final string follows a neat and regular structure. These problems often hide a simple repetition or cycle, and the task is to discover that cycle and ensure that the completed series aligns perfectly with it.

Given Data / Assumptions:

• Incomplete pattern: c _ e _ c d _ f _ d e _.• There are five blanks, and each option supplies exactly five letters.• The letters c, d, e and f appear repeatedly in the fixed positions.• We expect the completed series to look like a simple repeating block.

Concept / Approach:
Because the fixed letters already suggest segments like cde and cdef, we suspect the entire pattern might repeat some short sequence, perhaps the four-letter block cdef. By inserting each option and then examining the resulting full string, we can see whether a clean repetition emerges. The correct option typically produces a final string that can be divided into equal-length identical segments.

Step-by-Step Solution:
Take the template c_e_cd_f_de_ and fill the blanks with option a, dfecf.The completed series becomes: cdefcdefcdef.Now observe that this is simply the block cdef repeated three times.cdef | cdef | cdef forms a perfectly regular repetition.This strong regularity is a typical indicator of the intended solution in reasoning patterns.

Verification / Alternative check:
If we attempt to fill the blanks with the other options, the resulting strings look irregular and cannot be neatly partitioned into identical segments. For example, alternative fillings produce mixed orders like cdefcdcfedef or cdefcdffcded, which do not demonstrate any simple repeated block. Such patterns are unlikely to be used in well-designed aptitude questions, where a clear and simple rule is preferred.

Why Other Options Are Wrong:
Option b, c and d all create completed strings where the order of c, d, e and f is inconsistent and does not cycle cleanly. They might produce some local similarities, but when you examine the entire string, consecutive segments do not match. The absence of a clear repeated unit makes these options logically inferior to option a, which gives a very tidy triple repetition of the same four-letter block.

Common Pitfalls:
Candidates sometimes stop after finding a local match near the beginning of the string and do not check whether the end of the string follows the same pattern. Others may focus on counting how many times each letter appears, instead of verifying their order. For series completion tasks, always check the whole pattern and look for a concise, global rule, such as a repeating block.

Final Answer:
dfecf