Correct answer: A). Root
The part of the word that can not be broken down is called root word. In the word claustrophobic ''phob'' is the root word. It can not be suffix as a suffix are added after the root word. It cannot be prefixed as it is added before the root word. Here the prefix would be ''claustro'' and suffix would be ''ic''. It cannot be affixed as it contains both suffix and prefix.
Hence, the correct answer would be option A.