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

The total number of letters including blanks is 12. Let's divide the series into equal groups of 4 letters:
a _ c _  |  a _ c b  |  _ c c b
Let's check the letters at corresponding positions across the groups to find a uniform repeating pattern:

Comparing the second group (a _ c b) and the third group (_ c c b), the last two elements are c b.

Thus, the first group must also end with c b, making it: a _ c b→a b c b (putting b in the second gap).

Now the standard pattern group is determined as: a b c b.

Let's complete all groups using a b c b:

Group 1: a [b] c [b]

Group 2: a [b] c b

Group 3: [a] c c b (Notice: The options indicate another slight variation where it follows a shifting pattern, let's substitute option C (bdac):

a [b] c [d] a [a] c b [c] c c b→Not uniform.
Let's test option A (cbca):

a [c] c [b] | a [c] c b | [a] c c b
Here, the series is divided into groups of 4: a c c b repeats perfectly.

Group 1: a [c] c [b]

Group 2: a [c] c b

Group 3: [a] c c b

The inserted letters are: c, b, c, a.
Therefore, option A is correct.