Answer: In the word "acrophobic", "phob" is the root word.
Explanation: A root word is a word or morpheme that can act as a base to form new words. Moreover, prefixes and/or suffixes can be attached to root words. In the case of the word "acrophobic", "phob" is the root word and it makes reference to an abnormal fear. On the other hand, "acro-" is a prefix and "-ic" is a suffix. Both of them have been attached to the root word "phob".