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

The total length of the sequence including gaps is 12 letters. Let's test dividing it into four groups of 3 letters each: ab_ | d_b | b_a | _bd
Let's check option C (badb):

First blank: b → abb

Second blank: a → dab

Third blank: d → bda

Fourth blank: b → bbd
This does not form a repeating sequence.

Let's test option A (badd):

Sequence becomes: ab + b + d + a + bb + d + a + d + bd → abbdabbdabbd

Let's check the repeating pattern:
abbd | abbd | abbd

The pattern perfectly repeats the 4-letter block abbd. The letters filled in the gaps are b, a, d, d.