Complete the letter series by filling the blanks left-to-right. Rule: Each "_" stands for exactly one missing letter. Preserve the given case. Series: b _ y _ _ by _ b _ yt

Introduction / Context:
This question belongs to the “letter and symbol series” family. The goal is to infer a deterministic rule from the visible letters and fill each gap group with exactly one character, moving left-to-right. The completed string must be consistent with the smallest, repeatable pattern that explains all fixed letters.

Given Data / Assumptions:

• Stem: b _ y _ _ by _ b _ yt
• Each gap group “_” accepts one letter (there are five gap groups total).
• Case must match the stem (all lowercase where blanks appear).
• Options supply five letters: one for each gap group, in order.

Concept / Approach:
Prefer the simplest repeatable alternation interleaving the fixed anchors “b”, “y”, “…by…”, “…b…yt”. We look for a cadence that: (1) reproduces “b…y…by…b…yt” with regular spacing, and (2) does so using a single-cycle or short-cycle interleaving across all five fills.

Step-by-Step Solution:
1) Label five blanks as X1..X5.2) Try a minimal alternation that keeps “b” and “y” in phase: insert “t” after each “y” anchor and “b” after segments preceding “y”.3) Candidate sequence “btbtb” yields: btytb by t b b yt, which aligns a regular pattern of “t/b” fills around the fixed ‘b…y…by…b…yt’ scaffolding.4) No conflicts with fixed letters; cadence remains consistent across all five placements.

Verification / Alternative check:
Alternative strings (e.g., “bgtbt”, “atbbt”, “cbbte”) either introduce extraneous letters that fail to form a single repeating alternation or break adjacency regularity near “…by…” and “…yt”.

Why Other Options Are Wrong:

• bgtbt: “g” has no supporting repetition; breaks minimal-cycle principle.
• atbbt: Initial “a” misaligns the established “b…y” cadence.
• cbbte: Final “e” disrupts the “…yt” cadence and introduces unused letter class.

Common Pitfalls:
Counting underscores as total missing characters instead of gap groups; assuming multi-letter inserts per group; ignoring left-to-right mapping of options.

Final Answer:
btbtb