Fill with one ordered 5-letter string: _bcab_a_cab_a_c.

Introduction / Context:
This is a straightforward completion task where only one option has the required length to fill all blanks. We must still verify that it produces smooth overlaps consistent with “bcab” and “cab” tiles.

Given Data / Assumptions:

• Skeleton: _ b c a b _ a _ c a b _ a _ c
• Number of blanks: 5.
• Options lengths: acbcb (5), aabc (4), bbca (4), abac (4).

Concept / Approach:
Only “acbcb” can populate all five blanks. We still check adjacency to ensure it doesn’t break the visible “bcab / cab” motifs.

Step-by-Step Solution:
1) Insert a, c, b, c, b into the five blanks left-to-right.2) The resulting joints produce “…a b c a b c a c a b c a b a c…”, which keeps “bcab” and “cab” intact at their anchor positions.3) Shorter options cannot fill all blanks; therefore they are invalid by format.

Verification / Alternative check:
Scan for any doubled letters that would contradict the skeleton’s alternation—none occur.

Why Other Options Are Wrong:
They have only 4 letters and cannot fill 5 blanks, violating the problem’s structure.

Common Pitfalls:
Not counting blanks before choosing.

Final Answer:
acbcb