The correct option is A abdcb
Given the sequence a_abbb_ccccd_ddccc_bb_ba
By checking the options, we can clearly see that . only option C has a meaningful sequence.
After substituting Option C . the sequence will be ′aaa/bbbb/cccc/dddd/cccc/bbbb/a′ which is cyclic.The cycle contains elements in the
Hence the correct option is C(abdcb)