Fill four letters so the “a—baa—aaba—ca—b” string keeps its internal “aa” growth without breaking a/b balance:

Introduction / Context:
The fragments “baa”, “aaba”, and “ca” indicate increasing “aa” presence while alternating with b and c to avoid long monotone runs. The four-letter fill must preserve that gentle growth and land on “b” cleanly.

Given Data / Assumptions:

• Pattern: a — baa — aaba — ca — b
• Four letters inserted.
• No long identical clusters.

Concept / Approach:
We want a front “ab” to cue the first “baa”, then “a” to support the “aaba” inflow, and “c” to connect to “ca”, leaving the final “b” intact.

Step-by-Step Solution:

Verification / Alternative check:
Other choices either overuse c or create a-heavy blocks too early.

Why Other Options Are Wrong:

• bcca / ccaa: Introduce too much c or a at the wrong time.
• acaa: A-heavy at the tail, spoiling “…ca…b”.
• None of these: Not applicable.

Common Pitfalls:
Ignoring that “aaba” should feel like a gentle increase from “baa”.

Final Answer:
abac