Select the letter-cluster that can replace the question mark (?) in the following series.

pj, mg, jd, ga, dx, ?

Given: pj, mg, jd, ga, dx, ?

p - 3 → m
m - 3 → j
j - 3 → g
g - 3 → d
d - 3 → a 
Next first letter = a
For 2nd Letters:
j - 3 → g
g - 3 → d
d - 3 → a
a - 3 → x (Wrap-around in the alphabet)
x - 3 → u 
Next second letter = u
Missing letter-cluster = "au"
Thus, the Correct option is (c) au