What will come in place of question mark (?) in the following series

BDF, ?, PRT, WYA

Given: BDF, ?, PRT, WYA

Logic: Each letter is increase by 7.

First letter of all words; B?PW

B + 7 = I, I + 7 = P, P + 7 = W

Second letter of all words; D?RY

D + 7 = K, K + 7 = R, R + 7 = Y

Third letter of all words; F?TA

F + 7 = M, M + 7 = T, T + 7 = A

So, IKM is next term.

Thus, the correct answer is (b).