<em>Unpredictable: characterized by rapid and random change.</em>
Explanation:
Unpredictable is an adjective whose meaning lies in the fact, which we can see in the word itself, that something can not be predicted - a stable occurence transforms/modifies in an unexpected way in a short period of time which makes it hard to be prepared for it.
Other given choices do not resemble the given description in any way.
