Choose the 4-letter sequence that keeps the “aa … a … b … abaa …” cadence intact, avoiding triple clashes: aa__a__b__abaa__

Introduction / Context:
The anchors “aa … a … b … abaa …” indicate a controlled alternation where a’s dominate but b’s interject to prevent long a-runs. We must insert four letters to keep this cadence clean.

Given Data / Assumptions:

• Template: aa __ a __ b __ abaa __
• Four letters inserted in order.
• Avoid “aaaa” or “bbbb”.

Concept / Approach:
We want a brief b-anchor after the leading a’s, then another b near “abaa”, closing with an a for smoothness.

Step-by-Step Solution:

Verification / Alternative check:
Other options mis-time b’s (creating “bbb”) or leave long a-runs.

Why Other Options Are Wrong:

• abab / baba / baab: Each creates a harsher alternation that collides with anchors.
• None of these: Not required.

Common Pitfalls:
Overcorrecting with too many b’s.

Final Answer:
abba