_ stt _ tt _ tts _

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 gfeii/gfeii/gfeii/gfeii. Thus, the pattern 'gfeii' is repeated.

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.