Fill the blanks to complete: a _ b a _ b _ b _ a _ b.

Introduction / Context:
This is a binary alphabet pattern (a/b) with five blanks. The goal is to sustain the recurring “a b a / b …” rhythm without generating quadruple repeats or broken alternations.

Given Data / Assumptions:
Skeleton: a _ b a _ b _ b _ a _ b (5 blanks). Options: abaab, aabba, bbabb, abbab.

Concept / Approach:
We search for an option that (i) forms “a b a” near the start, (ii) keeps a double-b cluster only where the skeleton already demands it (“…b b…”), and (iii) closes the tail “…a _ b” neatly.

Step-by-Step Solution:
1) Insert “a b b a b” left-to-right.2) Early segment becomes “a a b a…”, then mid-sequence supports the required “…b b…” cluster.3) The tail resolves as “…a b”, preserving the alternation cadence.4) Other options either break the mid “bb” requirement or create awkward “aa…” runs at wrong locations.

Verification / Alternative check:
Rescan with 2-letter windows to confirm “ab/ba” alternation and one intended “bb” cluster.

Why Other Options Are Wrong:
They misplace the double-b or create “aa”/“bbb” sequences where the skeleton does not support them.

Common Pitfalls:
Fitting the first three blanks only; the crucial constraint is the mid “…bb…” region.

Final Answer:
abbab