Solution 26

The first phoneme of perfect, balance and mature is bilabial, and so is the last phoneme of –im. That is to say, in these words -im is more similar to /p/, /b/ and /m/ than /n/ would be. So the prefix has been assimilated, has been made more similar to the following sound, thus facilitating pronunciation. This has not happened in the remaining words, indirect, incorrect and insecure, as /d/, /k/ and /s/ are not bilabials