Find the odd letter block (positional math): Choose the string whose letters occupy square-number positions in the alphabet (1, 4, 9, 16, 25).

Introduction / Context:
Letter-series classification often relies on arithmetic applied to alphabet positions (A=1, B=2, …, Z=26). A particularly distinctive pattern is selecting letters that lie exactly at square-number positions in the alphabet, a property that stands out against “near-miss” or arbitrary jumps.

Given Data / Assumptions:

• Alphabet indices: A=1, D=4, I=9, P=16, Y=25 are perfect squares.
• Other strings include mixed or inconsistent jumps (not all-square positions).

Concept / Approach:
Map each letter to its numeric index and test for membership in {1, 4, 9, 16, 25}. The string that exclusively uses those indices exhibits a unique, coherent pattern.

Step-by-Step Solution:

Verification / Alternative check:
Inspect differences between successive letters: ADIPY follows +3, +5, +7, +9 (odd increments), another recognizable signature consistent with square steps from 1→4→9→16→25.

Why Other Options Are Wrong:
They mix increments that do not map to consecutive square positions, breaking the intended rule.

Common Pitfalls:
Do not rely solely on visual similarity; always convert to numeric positions to reveal the hidden structure.

Final Answer:
ADIPY