​Select the letter-cluster from among the given options that can replace the question mark (?) in the following series.

ADG, BEH, CFI, ?

Given: ADG, BEH, CFI, ?

Logic: Each and every letter of the pairs is increasing by +1 in alphabetical order.

First letter of every pairs;

A + 1 = B, B + 1 = C, C + 1 = D,

Second letter;

D + 1 = E, E + 1 = F, F + 1 = G,

Third letter; 

G + 1 = H, H + 1 = I, I + 1 = J,

Now, next word will be; DGJ

Thus, the correct answer is (b) DGJ