The correct answer is D. mob - move.
The root is the basic part of a word which is used to create other words by adding prefixes and suffixes to it. Here, that tiniest part of the word immobilize is mob, which comes from Latin and means to move. By adding the prefix im- you created the negative form of the verb, and by adding the suffix -ilize, you created that particular verb.
Images of bygone summers at the river eased through her mind like a movie in slow motion.
