Fill the four missing letters (left to right) so that the long series cycles cleanly. Series: L U _ T U P L U B T U _ L U B T _ P _ U B T U P

Introduction / Context:
This is a classic long-cycle restoration problem. We suspect a repeating 6-letter block due to the recurring L, U, B, T, U, P fragments.

Given Data / Assumptions:

• Known fragments strongly hint at the block L U B T U P.
• Four letters must be threaded to respect that block end-to-end.

Concept / Approach:
Assume the 6-letter cycle L U B T U P and check which fill completes every partial block into that same sequence without contradiction.

Step-by-Step Solution:

Verification / Alternative check:
Mark 6-character windows and confirm they all read L U B T U P.

Why Other Options Are Wrong:
They create at least one local violation of the L U B T U P order.

Common Pitfalls:
Misplacing letters by reading blanks out of sequence.

Final Answer:
BPUL