Correct Answer: cabaa

Solution : Given:
ab_cba_bcc_aabccb_ _bccba

To fill the series we have to divide the series – ab_cba / _bcc_a / abccb_ / _bccba
Let's check the option –
First option: abbac; ab a cba / b bcc b a / abccb a / c bccba (No repeated pattern has been found.)
Second option: ccaab; ab c cba / c bcc a a / abccb a / b bccba (No repeated pattern has been found.)
Third option: abcab; ab a cba / b bcc c a / abccb a / b bccba (No repeated pattern has been found.)
Fourth option: cabaa; ab c cba / a bcc b a / abccb a / a bccba (abccba is repeated in the series.)

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