Which of the following is an example of a run-on sentence

The run-on sentence is: I received a surprise in the mail today, I can't believe it.

It contains two independent clauses. I received a surprise in the mail today and I can't believe it can each stand alone without needing the other to finish its thought.

They can appear in the same sentence if they are separated by a semicolon; however, a comma tells the reader that they are part of the same idea, which is confusing.

answer; (b); the abolitionist movement;