Fill the blanks so the overall letter run follows a single consistent repeating pattern. Series: a a _ a a b b _ b _ a a _ a a b b _ b b

Introduction / Context:
In letter-series completion, multiple blanks usually mask a short repeating unit. Identifying the smallest repeating block makes it straightforward to test which fill restores uniform periodicity across the whole line.

Given Data / Assumptions:

• Series with five blanks: a a _ a a b b _ b _ a a _ a a b b _ b b
• Exactly one option inserts five letters (left to right) to yield one global cycle.

Concept / Approach:
Try each candidate, then partition the completed string into equal blocks. A 5-letter cadence is plausible because the distribution of a's and b's suggests clusters of two and three.

Step-by-Step Solution:

Verification / Alternative check:
Sliding a window of the candidate block across yields consistent overlaps with no contradiction.

Why Other Options Are Wrong:

• bbbaa, bbbba, babba: each creates at least one segment where the next chunk cannot start with the expected letter, breaking uniformity.

Common Pitfalls:
Testing only the first few positions and overlooking end-to-end periodicity, or inserting letters out of order.

Final Answer:
aabbb