Three of the following four letter-clusters are alike in a certain way and one is different. Pick the odd letter-cluster out.

Logic: 1st + 2 = 2nd, 2nd + 2 = 3rd.

Option (a): BDF 

B (+2) → D (+2) → F

Option (b): GIK

G (+2) → I (+2) → K

Option (c): PRT 

P (+2) → R (+2) → T

Option (d): EHK

E (+3) → H (+2) → K

EHK is the odd one out.

Thus, the correct option is (d) EHK.