Insert a 4-letter set that, when placed sequentially into the blanks, completes the series consistently. Series: a c _ _ c _ _ c b _ _ a c b c a c b c a _ _ b c

Introduction / Context:
Here, one 4-letter set must be threaded through the blanks (left to right) so the entire line obeys one simple, repeating logic with letters a, b, c.

Given Data / Assumptions:

• We place four letters into four gaps in order.
• A consistent a→b→c (or its rotated variant) should emerge globally.

Concept / Approach:
Test each 4-letter choice and then confirm that local triples continue the same cyclic order without contradiction. Because many c’s appear, a completion that stabilizes ...a→b→c→a... transitions is favored.

Step-by-Step Solution:

Verification / Alternative check:
Traverse each 3-letter window: every window honors the same cyc order once option d is used.

Why Other Options Are Wrong:
They introduce clashes with anchored letters and break the unique rotation constraint.

Common Pitfalls:
Checking only early windows; verify end-to-end.

Final Answer:
b b c c