In the following question, which one set of letters, when sequentially placed in the gaps in the series a_b_a_c_a_bc, will complete the letter pattern correctly?

Introduction / Context:
This question presents an incomplete sequence of letters with several blanks and asks which sequence of letters should be inserted, in order, to complete the pattern. The given base series is a_b_a_c_a_bc and we need to choose a five letter string that fills the underscores to create a meaningful and consistent pattern. Such problems test attention to local patterns and repetition in letter arrangements rather than pure alphabetic progression.

Given Data / Assumptions:

• Base pattern: a_b_a_c_a_bc • There are five blanks to be filled in order from left to right. • Options (five letter fills): bccbb, bcbca, cabca, acbcb • After substitution the final string must show a stable internal pattern, not just random letters.

Concept / Approach:
The approach is to try each option as a candidate fill and then inspect the resulting complete string for a simple, repeatable structure. We look for repeated blocks, symmetry, and a clear rule such as alternating positions with a certain letter. A correct option usually creates a sequence that can be seen as repeated chunks or a smooth transformation, while an incorrect option gives a more chaotic or inconsistent arrangement.

Step-by-Step Solution:
Step 1: Write the base framework with positions: a _ b _ a _ c _ a _ b c. Step 2: Insert option a) bccbb into the blanks, left to right. Step 3: The complete string becomes abbcaccbabbc. Step 4: Now examine the pattern in blocks of four letters: abbc, accb, abbc. The first block abbc is repeated in positions 1 to 4 and 9 to 12, with the middle block accb showing a similar structure where the two middle letters are both c. Step 5: The sequence now displays a controlled repetition where the outer letters of each four letter block are a and c or a and c like combinations, and the b and c in the middle shift in a regular way, indicating a designed pattern instead of randomness. Step 6: Try the other options quickly. With bcbca, cabca, or acbcb, the resulting full strings do not produce a neat repetition of blocks and give more irregular transitions. Step 7: Therefore, the option that yields a stable, repeated block pattern is bccbb.

Verification / Alternative check:
A different way to verify is to note that, after filling with bccbb, positions 1 to 4 and 9 to 12 are identical (abbc and abbc), which is a strong sign of a deliberate pattern. The central part accb nicely mirrors the same outer letters a and b while flipping middle characters to c, maintaining visual and positional regularity. None of the other candidates generate such clean, repeated four letter segments, which makes bccbb the unique consistent completion.

Why Other Options Are Wrong:
Option b) bcbca results in abbcabccaabc, where the blocks do not repeat cleanly and the internal structure appears irregular. Option c) cabca gives acbaabccaabc, breaking symmetry between beginning and ending segments. Option d) acbcb produces aabcabccabbc, which also lacks clear repeating four letter blocks and therefore seems arbitrary.

Common Pitfalls:
Many candidates simply test the first few characters and stop, without looking at the entire resulting string. Others try to force an alphabetic progression where the question is actually about repeating patterns of short letter blocks. It is important to fill all blanks with each option and then scan the full sequence for repetition and symmetry before deciding.

Final Answer:
The set of letters that completes the pattern a_b_a_c_a_bc is bccbb.