In the given context, the meaning of the word 'disorder' is : an abnormal condition. The main feature of the prefix dis- is that in makes word to function in an opposite meaning. Since the sentence is about the nervous system, the only logically and contextually correct option is the last one, as it represents negative meaning opposing to the needed condition. It is not a procedure, not an organ and there is no context clues that point out on it as illness. Also, disorder of nervous system is a stable biological term.
