In the word claustrophobic, phob is the root. The correct answer is C. It cannot be a suffix, because suffixes are added after the root, and here, the suffix is -ic. It also cannot be a prefix, because prefixes are added before the root, so here the prefix would be claustro-. It cannot be an affix, because affixes include suffixes and prefixes. Hope I helped! :) Cheers!
