Select the option that represents the letters that, when sequentially placed from left to right in the blanks, will complete the letter series.
_ c b _ _ a mc c _ e a a _c c _ e _ _m

Let's count the total number of letters and blanks in the series:
Total count = 24 elements.
We can break it into 4 sub-groups of 6 letters each:
_ c b _ _ a | m c c _ e a | a _ c c _ e | _ _ m _ _ _ -> Let's test a better group pattern.

Alternatively, let's substitute the options to find a meaningful repeating sequence.
Testing Option D: c e a b m b a a
The blanks filled sequentially give:
c c b e a a | m c c b e a | a m c c b e | a a m a b m (Does not create a smooth repetition)

Let's carefully observe the structure of the pattern:
The given series can be divided into groups of 6:
Let's check Option A: c e a b m b e a
Blanks: 1st=c, 2nd=e, 3rd=a, 4th=b, 5th=m, 6th=b, 7th=e, 8th=a
The completed text becomes:
c c b e a a | m c c b e a | a m c c b e | e a m...

Let's analyze cyclical shifts or variations:
Notice the sequence containing m c c _ e a. If the pattern is m c c b e a, then the letters used are m, c, c, b, e, a.
Let's check if each group of 6 is a permutation or a shifting pattern of a, b, c, c, m, e:
Group 1: c c b e a m (using the letters) -> Let's check Option D again but with correct alignment:
Blanks are 8 in total:
1 c b 2 3 a | m c c 4 e a | a 5 c c 6 e | 7 8 m ... wait, let's re-verify the input sequence length.
Given text: _ c b _ _ a m c c _ e a a _ c c _ e _ _ m
Let's index the blanks:

_ (before c)

_ (after b)

_ (before a)

_ (between c and e)

_ (between a and c)

_ (between c and e)

_ (after e)

_ (before m)
Total blanks = 8. Total letters = 14. Total = 22 characters.

Let's divide into groups of 6 or cyclic repetitions.
Let's plug option D (c e a b m b a a) into the 22-character string:
1=c, 2=e, 3=a, 4=b, 5=m, 6=b, 7=a, 8=a
Result: c c b e a a | m c c b e a | a m c c b e | a a m
Let's look at the groups:
Group 1: c c b e a a
Group 2: m c c b e a
Group 3: a m c c b e
Group 4: a a m ... (next would be c c b e)

Notice the shifting pattern:
In each group of 6, the elements shift to the right, and a new letter enters or cycles:

c c b e a a -> m enters at the front, others push right: m c c b e a

m c c b e a -> a enters at the front, others push right: a m c c b e

a m c c b e -> a enters at the front, others push right: a a m c c b

This perfectly forms a logical shifting/rolling series!
Therefore, the letters filled are indeed from Option D: c e a b m b a a.