Find the odd one from the given options which is different from others.

Looking at the alphabetical positions of each letter pair:

Option 1: P-K

P is the 16th letter, K is the 11th letter

Position relationship: $16 = 11 + 5$

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Option 2: T-F

T is the 20th letter, F is the 6th letter

Position relationship: $20 = 6 + 14$

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Option 3: O-L

O is the 15th letter, L is the 12th letter

Position relationship: $15 = 12 + 3$

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Option 4: R-I

R is the 18th letter, I is the 9th letter

Position relationship: $18 = 9 + 9 = 2 \times 9$

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Analyzing the pattern:

For P-K: $16 + 11 = 27 = 3^3$

For O-L: $15 + 12 = 27 = 3^3$

For R-I: $18 + 9 = 27 = 3^3$

For T-F: $20 + 6 = 26 \neq 27$

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In Options 1, 3, and 4, the sum of the alphabetical positions equals 27.

In Option 2 (T-F), the sum equals 26, which is different from the pattern.

Therefore, Option 2 (T-F) is the odd one out.