In the letter series d _ b a _ c _ a _ c b _, which set of letters, when placed sequentially in the blanks, will correctly complete the pattern?

Introduction / Context:
This is another missing letter series that uses the letters a, b, c, and d. The goal is to identify one sequence of five letters which, when inserted into the blanks, restores a structured pattern throughout the entire series. Such questions typically encode a repeated block of characters, and solving them requires careful observation and an eye for symmetry and repetition.

Given Data / Assumptions:

• Incomplete series: d _ b a _ c _ a _ c b _.
• There are five blanks to be filled in order.
• Each option provides exactly five letters to occupy these positions.
• The final result should display a consistent pattern, ideally a repeated block.

Concept / Approach:
The letters d, c, b, and a suggest that the pattern might be based on a descending order such as d c b a. We consider the possibility that the entire series is constructed by repeating the block d c b a three times. By filling the blanks according to each option, we see which one produces the desired repetition with no contradictions. The correct option should give a series where the same four letter block is visible across the entire string.

Step-by-Step Solution:
Step 1: Identify the positions of the blanks: 2, 5, 7, 9, and 12. Step 2: Choose option A, which gives the letters c d b d a. Step 3: Insert them to obtain the full series: d c b a d c b a d c b a. Step 4: Group this as dcba dcba dcba. Step 5: Each block dcba is identical and the entire line is composed of three such blocks. Step 6: This confirms that option A restores a perfect repetition of the dcba pattern.

Verification / Alternative check:
To verify, read off each group of four letters: positions 1 to 4 form dcba, positions 5 to 8 form dcba, and positions 9 to 12 form dcba again. There is no mismatch in the order of letters within any block. If another option is tried, the resulting sequence will not decompose into such neat repeated dcba segments, making option A uniquely correct. This direct block comparison is an efficient way to check the solution.

Why Other Options Are Wrong:
Option B (cdbad) introduces an a at a place where the descending dcba order would expect a different letter, thereby destroying the repeated pattern. Option C (bdacd) results in irregular sequences with unexpected letter orders, so it cannot be partitioned into identical blocks. Option D (abdca) also fails to produce three dcba blocks and instead creates mixed arrangements that do not show a simple cycle. These irregularities mean that none of these options match the clear descending repetition found with option A.

Common Pitfalls:
Exam takers sometimes focus only on the first few letters, see a partial descending sequence, and then commit to an option without verifying the rest of the series. Another common error is neglecting to regroup the letters into equal sized blocks for comparison. A systematic approach that examines the entire series and explicitly checks for repeating units usually makes the correct pattern stand out and avoids incorrect selection based on partial fits.

Final Answer:
The correct set of letters is option A, cdbda, which yields the repeated series dcba dcba dcba.