Answer:
shortening
Explanation:
Let's first briefly define each of these processes.
Prefix and suffix addition is, as the name suggests, adding prefix or a suffix to a word to make a new word. (terrain -> subterrain; escalate -> escalator)
Compounding is basically making a new word from two independent words (sun + flower -> sunflower)
Telescoping, simply put, is blending two words into one (smoke + fog -> smog)
Shortening is just cutting out a part of the word to make word, imagine that, shorter. The new word stays the same part of speech as the original one and is mostly used as a slang. (laboratory -> lab)
Back formation must not be mixed with shortening. Words are usually made from adding affixes to a stem. That way from one stem we can make many words with different meanings (play -> player, playing, replay, outplay, playroom, playful...).
Back formation is an opposite process. It's making shorter words from original, longer words by removing affixes. (paramedical -> paramedic).
Note that, in back formation, new and original word may differ in part of speech they belong to and in meaning , unlike in shortening. Also, in back formation, words are shortened strictly by removing affixes; in shortening, there are no rules.
So, finally, for the word phone. It derives from the word telephone, it represents its shorter version. Since both, phone and telephone, are the same part of speech, either a noun or a verb, and they denote the same thing, it is considered that the process of shortening was used to form the word phone.
