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
Option 2: T-F
T is the 20th letter, F is the 6th letter
Position relationship: 20=6+14
Option 3: O-L
O is the 15th letter, L is the 12th letter
Position relationship: 15=12+3
Option 4: R-I
R is the 18th letter, I is the 9th letter
Position relationship: 18=9+9=2×9
Analyzing the pattern:
For P-K: 16+11=27=33
For O-L: 15+12=27=33
For R-I: 18+9=27=33
For T-F: 20+6=26=27
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.