gfe _ ig _ eii _ fei _ gf_ ii

The series is tst/tst/tst/tst. Thus, the pattern 'tst' is repeated.

Each term consists of consecutive letters in order. The number of letters in the terms goes on increasing by one at each step.

Also, there is a gap of one letter between the last letter of the first term and the first letter of the second term; a gap of two letters between the last letter of the second term and the first letter of the third term; and so on.

So, there should be a gap of three letters between the last letter of the third term and the first letter of the desired term.

The series is baa/bba/baa/bba/baa/bb. Thus, the pattern baa/bba is repeated.

The series is abc/aabc/aabbc/aabbcc/a.

The series rtus/rtus/rtus/rtus. Thus, the pattern 'rtus' is repeated.

The series is ab/ab/ab/ab/ab/ab. Thus, the pattern 'ab' is repeated.

The series is cabbbb/cabbbb/cabbbb. Thus, the pattern 'cabbbb' is repeated.