Choose the 7-letter string that completes the pattern: _b_baaabb_a_ _bb_a_.

Introduction / Context:
This series uses only a and b with visible clusters like “baaabb” and “bb”. We must insert one 7-letter option to make all segments align without creating contradictions like “aaaa” or “bbbb”.

Given Data / Assumptions:

• Skeleton (spaces show groups): _ b _ baaabb _ a _ _ bb _ a _
• Options (7 letters): abbaaba, ababbaa, babaaba, baabaab.

Concept / Approach:
We look for a candidate whose internal sub-blocks (e.g., “baa”, “aab”, “abb”) naturally splice into “…baaabb…” and later “…bb…a…”. Among classical patterns, “baabaab” often tiles cleanly with “baa/ab/abb” overlaps.

Step-by-Step Solution:
1) Insert “b a a b a a b” left-to-right across the seven blanks.2) Early joints reinforce “…b b…” then “…baaabb…”, preserving the long middle cluster.3) Later joints align with the “…bb…” and final “…a” anchors without producing quadruple runs.4) Other options tend to create inversions (e.g., “…abab…” near “baaabb”) or misplace a double-b that splits the central cluster.

Verification / Alternative check:
Rescan with sliding windows to ensure that the prominent “…baaabb…” appears exactly once and that the fill does not fragment it.

Why Other Options Are Wrong:

• abbaaba / ababbaa / babaaba: Each introduces at least one conflicting junction that either splits the “baaabb” core or produces an excessive run of identical letters.

Common Pitfalls:
Optimizing for the first gap only; the critical constraint is preserving the “baaabb” core.

Final Answer:
baabaab