Fill the five underscores with a 5-letter sequence so the alternating “aba” blocks recur neatly across the whole string: _ _ aba _ _ ba _ ab

Introduction / Context:
The skeleton exhibits two anchors “aba” and “ba…ab”, suggesting an alternation of the “aba” motif. We must select a 5-letter fill that threads these anchors without creating “aaaa” or “bbbb”.

Given Data / Assumptions:

• Template: _ _ aba _ _ ba _ ab
• Five letters inserted in order.
• Avoid long identical runs.

Concept / Approach:
Choose a fill that starts by preparing the first “aba” anchor with a mild b/a cushion, cross the middle “ba” anchor without collision, then taper into the final “…ab”.

Step-by-Step Solution:

Verification / Alternative check:
Other options either cause “bbbb” or displace the “aba” cadence at the center.

Why Other Options Are Wrong:

• baabb: Mis-times the “aa” relative to the first anchor.
• abbbb: Creates “bbbb”.
• bbaba: Overloads b’s near the center.
• None of these: Not needed, “abbab” fits.

Common Pitfalls:
Fixing the front “aba” but breaking the “ba…ab” closure.

Final Answer:
abbab