Three of the following letter-clusters are alike in some manner and hence form a group.

Which letter-cluster does not belong to that group?

XFIK

1   2    3    4   5    6   7     8    9  10  11  12  13

 A   B   C    D   E    F   G    H    I    J    K    L   M

 ↓    ↓    ↓    ↓    ↓     ↓   ↓    ↓     ↓   ↓    ↓    ↓    ↓

 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

26  25  24  23  22  21  20  19  18  17  16  15  14

THE LOGIC FOLLOWED HERE IS;

(1st letter +12 → 2nd letter +5 → 3rd letter +5 → 4th letter)

So,

1) AMRW,

A → (+12) → M

M → (+5) → R

R → (+5) → W

2) XFIK,

X → (+8) → F

F → (+3) → I

I → (+2) → K

3) PBGL,

P → (+12) → B

B → (+5) → G

G → (+5) → L

4) KWBG,

K → (+12) → W

W → (+5) → B

B → (+5) → G

Thus, XFIK does not belong to that group.