lmn/nml/lmn/nml ? mnnm
The sequence is repeat of alternative two block lmn and nml.

Given word is combination of mbb and maa.
Fill the missing letters as per above two words.
mbb/maa/mbb/maa/mbb ? mabam

The pattern is abncb.
Fill the missing letters as per above word and pattern.
abncb/abncb/abncb ? bcabab

The pattern is baab. Now,
baab/baab/baab/baab ? bbaab

The sequence is made of two block cca and ccb alternately.
Fill the missing letters as per above words and pattern.
cca/ccb/cca/ccb/cca/ccb/c ? acacc

tst/tst/tst/tst ? tsst
The sequence is made of repetition of block tst.

aceg/aceg/aceg ? ecag
The series is made of repetition of block aceg.

accb/accb/accb/accb ? cbcc
The series is made of repetition of block accb.

The first word is bca. Then first position letter is moved to the last position in each steps.
bca/cab/abc/bca ? babca

The sequence is made of repetition of abc so identify the missing alphabet.
abc/abc/abc/abc/abc

Correct Answer is acbcb