Correct Answer: cbaca

Solution : Given:
ab_dda_ccd_bb_d_

To fill the series we have to divide the series – ab_d / da_c / cd_b / b_d_
Let's check the options –
First option: ccaac; ab c d / da c c / cd a b / b a d c (No repeated pattern has been found.)
Second option: cbaca; ab c d / da b c / cd a b / b c d a (In each part, the last letter is moved to the first place followed by the first letter at the second place, the second letter at the third place, and the third letter at the fourth place.)
Third option: cbcba; ab c d / da b c / cd c b / b b d a (No repeated pattern has been found.)
Fourth option: cbbcd; ab c d / da b c / cd b b / b c d d (No repeated pattern has been found.)

So, the series is→abcddabccdabbcda. Hence, the second option is correct.