Correct Answer: aabba

Solution : Given:
b _ ac _ cc _ cb _ ab _ ac

To fill the series we have to divide the series – b _ ac _ cc _ / cb _ ab _ ac
Let's check each option –
First option: cbaba ; b c ac b cc a / cb b ab a ac (No repeated pattern has been found)
Second option: bbaac ; b b ac b cc a / cb a ab c ac (No repeated pattern has been found)
Third option: abbbc ; b a ac b cc b / cb b ab c ac (No repeated pattern has been found)
Fourth option: aabba ; b a ac a cc b / cb b ab a ac (The pattern followed is – Add 1 to b and a; Subtract 2 from c)

So, the series becomes→ baacaccbcbbabaac . Hence, the fourth option is correct.