BCB, DED, FGF, HIH, ___

Answer: JKJ

Reasoning:

• First letters: B, D, F, H, ... each advances by two letters in the alphabet (B → D → F → H → J). So the next first letter is J.

• Second letters: C, E, G, I, ... also advance by two letters (C → E → G → I → K). So the next second letter is K.

• Third letter: each group repeats its first letter, so with first letter J the third letter is also J.

Combining these gives the next group: JKJ.