Restore a uniform p–q–r progression by filling the nine missing letters. Series: p q r _ _ r s _ r s _ _ s _ q _

Introduction / Context:
This item uses a limited alphabet {p, q, r, s} with a near-linear “step” behavior. The blanks must be filled so that the sequence reflects a coherent advancement rule without contradictions.

Given Data / Assumptions:

• Framework: p q r followed by multiple gaps interleaved with r, s, q markers.
• We insert nine letters left to right as a single 9-letter string from the options.

Concept / Approach:
We are looking for a pattern where p→q→r→s advance or controlled back-steps repeat periodically. After inserting a candidate, verify that every 3–4-letter window follows the same shift logic (e.g., +1 step or a mirrored +1, −1 cadence).

Step-by-Step Solution:

Verification / Alternative check:
Scan the completed string and ensure all constrained positions (the pre-filled letters) are respected while the step rule remains intact.

Why Other Options Are Wrong:

• They compel at least one illegal local transition (e.g., skipping or reversing two steps) relative to anchors.

Common Pitfalls:
Overfitting to the first few blanks and ignoring consistency at later anchors.

Final Answer:
sqprrqr