Letter-series completion – insert a 4-letter block to complete the cyclic pattern: _bcab_cabc_abca_b Which option, placed sequentially in the blanks, maintains a repeating abca cycle across the entire string?

Introduction / Context:
Cyclic letter patterns commonly use a short base block that repeats across the entire string. Here, the visible fragments strongly suggest the 4-letter cycle "a b c a", which is a simple loop returning to "a" after "c". Our job is to find which 4-letter option, inserted in order into the blanks, preserves a perfectly continuous repetition with no mismatches at the boundaries.

Given Data / Assumptions:

• Pattern (underscores indicate missing letters): "_bcab_cabc_abca_b"
• Observed visible runs include "... bcab ..." and "... cabc ...", which align with "... b c a b ..." and "... c a b c ...", typical subwindows of the repeating cycle "a b c a".
• We assume a single block fills all blanks in sequence.

Concept / Approach:
When a short cycle (length 4) underlies the entire string, each contiguous 4-letter window should be a rotation-consistent slice of the base block. The fragments "bcab" and "cabc" are both contiguous slices from repeating "abcaabca...". Thus, inserting "abca" in the blanks should reconstruct a seamless loop without any letter collisions or broken transitions at the join points.

Step-by-Step Solution:

Verification / Alternative check:
No boundary conflict occurs at the ends because the cycle returns to "a" after "c", aligning with the revealed trailing "b" once the preceding letters are in place.

Why Other Options Are Wrong:

Common Pitfalls:
Choosing a block that matches a single local fragment but breaks the global cyclic continuity elsewhere.

Final Answer:
abca