A b d g what comes next?

Answer 1

e

That's a guess, but it's a good guess. The first five letters of the Greek alphabet are alpha, beta, delta, gamma, and epsilon. Their initials are a, b, d, g, and e.

Another view:

K. (A B c D ef G hij K) Do I win a prize?

Answer 2

K

as in go four letters along

A + 1 = B

B + 2 = D

D + 3 = G

G + 4 = K

Answer 3

(B-D)