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

The total number of characters (including gaps) is 12. Let's try dividing the series into groups of 3 or 4:
Let's try groups of 3: d_c | _d_ | cb_ | c_b
Let's test Option D (c, b, d, b, c):

First gap: c → d c c

Second gap: b → b d d
This doesn't show a clear pattern.

Let's test Option B (c, b, c, d, c):

Replacing the blanks gives: d c c b d c c b d c c b

Let's break this resulting pattern into groups of 4:

d c c b

d c c b

d c c b

The pattern repeating is perfectly dccb repeated 3 times.
Therefore, the letters to be filled are c, b, c, d, c.