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

Let's check the options by substituting them into the gaps of the pattern d _ b a _ c _ a _ c b _:
Let's try Option B (cdbad):

Gaps filled sequentially: 1st=c, 2nd=d, 3rd=b, 4th=a, 5th=d

The complete string becomes: d c b a / d c b a / d c b a

We can clearly see it forms a highly consistent repeating cycle of 4 letters: d-c-b-a.

1st block: d c b a

2nd block: d c b a

3rd block: d c b a

Thus, the correct letter set is cdbad.