Complete the repeating block pattern by filling one 4-letter string: H_JH_IJHHI_HH_JH. Choose the set of four letters that, when placed in order, yields a consistent repeating sequence.

Introduction / Context:
This problem features a letter sequence with missing letters in multiple positions. The aim is to identify a short repeating block and supply a single 4-letter fill that restores the repeated pattern across the entire string. Such items test pattern discovery and global consistency checks, not just local neighbor-matching.

Given Data / Assumptions:

• Skeleton: H _ J H _ I J H H I _ H H _ J H
• We must choose one 4-letter sequence to replace the blanks in order (blank1 → blank2 → blank3 → blank4).

Concept / Approach:
Scan for a likely repeating unit. The visible chunks “H J H” and “I J H H I” suggest the motif HIJH or a near variant. If the complete string decomposes into HIJH blocks, each four-letter segment should read H, I, J, H cyclically.

Step-by-Step Solution:
1) Try to enforce HIJH as the repeating block: H I J H | H I J H | H I J H | H I J H.2) Fill blank1 with I to get the first full block HIJH starting at position 1.3) Fill blank2 with H so that the next letters maintain the cycle H I J H | H…4) Fill blank3 with I so the mid-run becomes …H I J H H I J H… consistent with repeated HIJH.5) Fill blank4 with J to complete the final HIJH block at the tail.6) The four inserted letters in order are I, H, I, J → “IHIJ”.

Verification / Alternative check:
After insertion, segment the line into four-letter groups: HIJH | HIJH | HIJH | HIJH. Each block matches perfectly, confirming uniform periodicity.

Why Other Options Are Wrong:

• IHJI / HIHI / HJHJ: Each produces at least one block with broken order or doubled letters at the wrong positions, destroying the clean HIJH repetition.

Common Pitfalls:
Solvers often fill blanks greedily (matching immediate neighbors) and miss the four-letter macro-cycle. Always hypothesize a block, then test the entire string for global fit.

Final Answer:
IHIJ