Choose the four-letter sequence to complete the pattern. Series: abca _ bcaab _ ca _ bbc _ a

Introduction / Context:
This problem involves restoring a repeating motif formed by the letters a, b, and c. The right completion must preserve the internal blocks like "abca", "bcaab", and "bbc" without creating impossible joins.

Given Data / Assumptions:

• Pattern skeleton: abca _ bcaab _ ca _ bbc _ a
• There are four blanks to be filled by a four-letter option in order.

Concept / Approach:
Detect the rolling presence of “abca” and “bca” motifs and ensure that the inserted string produces smooth overlaps while retaining “bbc” near the end.

Step-by-Step Solution:
Insert "abac": abca|a|bcaab|a|ca|b|bbc|a|a. The joins remain natural: “abca” repeats neatly, then “bcaab” and “ca” are respected, and “bbc” is untouched.

Verification / Alternative check:
Other choices create congestion (e.g., c merges) or misalign the “bca” clusters, breaking the recognizable motif.

Why Other Options Are Wrong:
ccaa: Overloads c near “bca” joins. bbaa: Warps the “abca/bca” cadence. abba: Misplaces b near the “bbc” anchor.

Common Pitfalls:
Ignoring anchor blocks at the end; the correct fill must preserve “bbc”.

Final Answer:
abac