Answer:
D. An abnormal condition.
Explanation:
The use of the prefix dis- signify the negative form of the word with which it is joined. So, when the prefix is used alongside "order' it becomes a new word "disorder". Disorder, then, means some form of irregularity, disaster or even something in a mess.
The sentence is-
The article in the medical journal described a disorder of the nervous system.
The word "disorder" as used in the sentence means "an abnormal condition" of the nervous system. A disorder in the sentence will mean that there is some sort of problem with the nervous system. So, the other options are invalid in relation to the sentence in which it is associated with.
