Fill with one ordered 4-letter string to complete the run: ac_bacc_a_cba_cb.

Introduction / Context:
This sequence is built from the letters a, b, c with stable tiles like “ac”, “bacc”, and “cba”. We must insert one 4-letter option to realize a coherent concatenation of these tiles.

Given Data / Assumptions:

• Skeleton: a c _ b a c c _ a _ c b a _ c b
• Options (4 letters each): cbcc, cbbc, caac, bbcb.

Concept / Approach:
We look for a fill that (i) closes “ac_ba” into “acb a…”, (ii) keeps “bacc” intact, and (iii) bridges into “…cba…” without creating clashes like “aa” or “bbb” where the pattern favors alternating around c.

Step-by-Step Solution:
1) Try “cbbc”: insert c then b to obtain “ac c b a c c…”, which keeps “acc” and then “bacc” contiguous.2) The third letter b sustains the b-centered mid segment, and the fourth letter c prepares the handoff into “cba”.3) Alternatives introduce inconsistencies: “cbcc” yields over-stacked c’s; “caac” creates double a’s in a sensitive zone; “bbcb” leaves an early b-b congestion that breaks the “ac/bacc” cadence.

Verification / Alternative check:
Re-scan to see standard sub-blocks “ac”, “bacc”, “cba” appearing with clean joints.

Why Other Options Are Wrong:

• cbcc: Too many consecutive c’s mid-run.
• caac: Produces “aa” misfit across an “ac/ca” boundary.
• bbcb: Forces b-clusters that mute the required c-anchors.

Common Pitfalls:
Greedy local fits that ruin later obligatory tiles like “cba”.

Final Answer:
cbbc