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

The total number of characters including blanks is 15. We can divide the pattern into groups of 3:
_cc | a_a | _ca | _ac | _ab

Let's try substituting Option B (a, b, c, b, b):

First blank → a : acc

Second blank → b : aba

Third blank → c : cca

Fourth blank → b : bac

Fifth blank → b : bab

Let's test Option C (a, b, b, c, c):

acc

aba

bca

cac

cab

Let's test a 5-letter repeating block pattern:
_cca_ | a_ca_ | ac_ab
With Option D (a, b, c, c, b):

acca_ → accab

a_ca_ → accab

ac_ab → accab

This forms a perfect repeating sequence of accab | accab | accab.