In each of the following groups, the three letters are related by a fixed…

Concept

In a letter-series "odd one out", convert each letter to its position in the English alphabet (A = 1, B = 2, …, Z = 26) and look for a constant relationship that links the three letters of every group. The group whose letters do not obey that common relationship is the odd one.

Application

Compare the second and third letters of each group with the first, using alphabetical positions:

1. F(6), B(2), I(9): from the first letter, the second is 4 places back (6 − 4 = 2) and the third is 3 places ahead (6 + 3 = 9). Rule: −4, +3.

2. E(5), A(1), H(8): 5 − 4 = 1 and 5 + 3 = 8. Rule: −4, +3.

3. I(9), E(5), L(12): 9 − 4 = 5 and 9 + 3 = 12. Rule: −4, +3.

4. R(18), N(14), U(21): 18 − 4 = 14 and 18 + 3 = 21. Rule: −4, +3.

5. P(16), L(12), R(18): 16 − 4 = 12 gives the correct second letter, but 16 + 3 = 19 = S, not R(18). The third letter is only +2 from the first instead of +3.

Cross-check

Four groups share the identical −4, +3 mapping from their first letter. The group P, L, R is the single one whose third letter is +2 (not +3) from the first, so it breaks the common rule and is the odd one out.