Correct Answer - Option 3 : cdegf
Checking options one by one:
1) bcd / ceef / ddfgh / efghi → doesn't follow any pattern
2) bcd / cdef / deffh / egghi → doesn't follow any pattern
3) bcd / cdef / defgh / efghi → follows a pattern
4) bbd / ccef / ddfeh / efghi → doesn't follow any pattern
Hence, "cdegf" is the correct answer.