The bolded pronoun in the given sentence is the word "which". This pronoun is an example of an interrogative pronoun. Interrogative pronouns are pronouns that are used to introduce a question. The antecedents of interrogative pronouns are unknown because it is what is being asked in the sentence that is missing.
<span>The correct answer is A. The river is constantly changing. The author describes the Mississippi as fickle because it is not constant - you can never know when it is going to change, and whether that change will be for the better or for the worse. The word fickle means changeable, volatile, irregular. </span>