In the following question, which one set of letters when sequentially placed at the gaps in the given letter series shall complete it? a b a _ b a d a _ b a _  b a d a a b a d _ a d a

d a d b d d a b d a b d a b d d

d a d b

a b a d b a d a / a b a d b a d a / a b a d b a d a