Correct Answer: bccdbcab

Solution : Given -

a_ _ dba _ _ bcad _ _ da _ _cd

Let's check the options.

option first: aabbccdd

The series becomes a aa dba bb bcad cc da dd cd

There is no pattern in the above series.

option second: bccdbcab

The series becomes a bc dba cd bcad bc da ab cd

The pattern in the series becomes abcd/bacd/bcad/bcda/abcd.

option third: abcddcba

The series becomes a ab dba cd bcad dc da ba cd

There is no pattern in the above series.

option fourth: cbcddcba

The series becomes a cb dba cd bcad dc da ba cd

There is no pattern in the above series.

Hence, second option is correct.