Complete the letter series by placing one set of letters sequentially in the gaps: ab_d_aaba_na_badna_badna Which option, used once from left to right, completes the series consistently?

Introduction / Context:
This problem asks for a single 5-letter sequence to be inserted into successive gaps so that the entire string reads smoothly with a repeating internal motif. The visible tail "... _badna_badna" strongly suggests the motif "badna". The task is to verify that the same set of letters, applied once from left to right, resolves all blanks while preserving local continuity and global repetition.

Given Data / Assumptions:

• Template: "ab_d_aaba_na_badna_badna" (underscores are gaps).
• Visible endings already include "...badna_badna", hinting at a repeated unit "badna".
• We assume exactly one 5-letter set is placed sequentially (no reordering within the set).

Concept / Approach:
When a concluding segment exhibits a clear repeating block, a robust approach is to test that block against earlier blanks. If the same letters, applied once in order, yield valid letter adjacencies (no doubled contradictions, no broken sub-words), then the block is the correct completion. Because "badna" already appears twice contiguously at the end, using "badna" to fill the remaining gaps ensures the earlier segments align with the same internal rhythm.

Step-by-Step Solution:

Verification / Alternative check:
Scan the final constructed string left to right and confirm that every adjacency matches the repeated motif and that no duplicated letters break expected order.

Why Other Options Are Wrong:

Common Pitfalls:
Picking a set that fixes the first gap nicely but fails at the later, already-fixed segments, especially near the tail where the motif is explicit.

Final Answer:
badna