Complete the long alternating pattern by choosing one ordered 6-letter set: _ c b _ c a _ b a c b _ c a _ b a c _ d.

Introduction / Context:
This is a dense placement problem with six blanks and a target alphabet {a, b, c, d}. The skeleton already contains a, b, c in a rhythm; the chosen option must introduce d’s at precise points while preserving the interleaving of a, b, c.

Given Data / Assumptions:

• Skeleton: _ c b _ c a _ b a c b _ c a _ b a c _ d
• Options (as 6 letters, spaced for readability): “b a d d d b”, “b b b d d d”, “a d d d d b”, “a d d b b b”.

Concept / Approach:
We must place d sparingly so that c–b and b–a alternations are not destroyed. Over-insertion of d (e.g., four or more in a row) tends to break the repeating bigrams already present in the host string.

Step-by-Step Solution:
1) Test “a d d b b b”: leading “a” closes the early “_ c b” into “a c b”, a natural motif.2) The two d’s in the middle create necessary transitions at positions that do not collide with “c b” and “b a” pairs.3) The trailing b b b consolidates the “…c a _ b a c…” zone without overloading d at the tail where a set “…_ d” is already present.4) Alternatives either overload d (four or more, causing disruptions) or place b’s where an a/c handoff is required.

Verification / Alternative check:
Walk the final line with bigram windows and confirm intact cycles among a, b, c while d appears as controlled pivots.

Why Other Options Are Wrong:

• b a d d d b / b b b d d d / a d d d d b: Each creates excessive d density, breaking established a/b/c alternations.

Common Pitfalls:
Counting letters but not testing bigram continuity.

Final Answer:
a d d b b b