Letter Series Analysis:
The given letter series is: ab _ bbc _ c _ ab _ ab _ b.

Pattern Analysis:
To solve this letter series, we need to identify the pattern and fill in the missing letters.

Looking at the series, we can observe the following patterns:
1. The series is divided into groups of three letters.
2. The first letter of each group is 'a', while the last letter of each group is 'b'.
3. The second letter of each group follows the pattern - 'b', 'c', 'a', 'a', 'a', 'b'.

Filling in the Missing Letters:
Now, let's fill in the missing letters based on the identified patterns:

1. ab _ bbc _ c _ ab _ ab _ b
2. ab b bbc c a ab ab b

Explanation of the Filled Letters:
1. The missing letter after 'ab' can be filled with 'b' as it follows the pattern of the second letter in each group.
2. The missing letter after 'bbc' can be filled with 'c' as it follows the pattern of the second letter in each group.
3. The missing letter after 'c' can be filled with 'a' as it follows the pattern of the second letter in each group.
4. The missing letter after 'ab' can be filled with 'a' as it follows the pattern of the second letter in each group.
5. The missing letter after 'ab' can be filled with 'a' as it follows the pattern of the second letter in each group.
6. The missing letter after 'ab' can be filled with 'b' as it follows the pattern of the second letter in each group.

Therefore, the correct letter series is: abbbbcacaaabb.

Choosing the Correct Option:
Now, let's compare the correct letter series with the given options:

- Option 'a' (ccaac): This option does not match the correct letter series.
- Option 'b' (cbabc): This option does not match the correct letter series.
- Option 'c' (cacac): This option matches the correct letter series.
- Option 'd' (bccab): This option does not match the correct letter series.

Hence, the correct answer is option 'C' (cacac).