The correct answer is -ic.
When you add the suffix -ic to the root phon, you get the word phonic, which means 'something that has the characteristics of sound.' If you add the suffix -al to phon to create phonal, it means something related to sound, which is not the exact same meaning (so that is a wrong option).
Words phonen and phonly don't exist, so -en and -ly are wrong options as well.