In the letter series CD_E_DD_CD_E, which set of letters, when placed sequentially in the blanks, will complete the pattern?

Introduction / Context:
This problem gives a partially hidden letter series CD_E_DD_CD_E and asks which four letter set, when filled into the blanks from left to right, completes a regular pattern. The letters involved are C, D, and E, and many such questions are built by repeating a short block several times.

Given Data / Assumptions:

• Template: C D _ E _ D D _ C D _ E.
• There are four blanks, to be filled by a four letter option in order.
• Options: CDCD, DCCC, DCED, DDDC, CCDD.
• We expect a simple repeating pattern after filling in the blanks.

Concept / Approach:
The standard approach is to try each candidate set, fill the blanks, and then look for repeating groups. When the correct option is inserted, the full series should break neatly into identical blocks. Because the visible letters C, D, and E already suggest a short block like CDDE, we check if any option causes the entire string to consist of that same block several times.

Step-by-Step Solution:
Step 1: Label the blanks as x1, x2, x3, x4. The pattern is C D x1 E x2 D D x3 C D x4 E. Step 2: Try option C, DCED, which gives x1 D, x2 C, x3 E, x4 D. Step 3: Substitute: C D D E C D D E C D D E. Step 4: Group the completed series into blocks of four letters: CDDE | CDDE | CDDE. Step 5: Each block is identical, so the sequence is clearly the block CDDE repeated three times.

Verification / Alternative check:
Check briefly that other options do not produce such a clean repetition. For example, using CDCD or DCCC leads to uneven groupings where at least one segment differs from the others, so you cannot write the entire sequence as a repetition of a single four letter block. Only DCED gives three exact copies of CDDE, which is a very natural intended pattern.

Why Other Options Are Wrong:
Option A CDCD produces mixed groups that do not align into equal repeated segments. DCCC and DDDC write multiple D letters together in ways that do not match the CDDE structure already suggested by the visible sections. CCDD cannot be grouped with the fixed letters to form identical blocks. Thus, they fail to produce a simple repeating pattern.

Common Pitfalls:
One common mistake is to stop after checking only one part of the line and choosing the option that fits locally, without testing the entire string. Another is to search for a numerical progression between individual letters, when the actual structure is based on repeating a short word like CDDE. For pattern completion questions, it is usually best to check whether the full sequence can be split into equal, identical blocks after filling the blanks.

Final Answer:
The set of letters that correctly completes the series is DCED, creating the pattern CDDE three times in a row.