I'm stuck at the fourth exercise of David Odden's Introducing Phonology. It's about underlying representations.
Link:

I have such a difficult task with finding underlying representations and the only pattern which I seem to notice is the consonant voicing or devoicing depending on the suffix. I came up with the following rule but I don't know if it's correct or if it covers the entire task: C ? voiced / _ voiced consonant
Could anyone lend me a hand with this?