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

A run-on sentence occurs when two or more independent clauses are improperly joined without appropriate punctuation or conjunctions. In the chosen example, the sentence "She enjoys reading books she goes to the library often" consists of two independent clauses: "She enjoys reading books" and "she goes to the library often." These clauses express complete thoughts and could stand alone as sentences, but they are incorrectly fused together without any punctuation or coordinating conjunction. This failure to separate the clauses clearly exemplifies a run-on sentence, making it the correct choice.

Other sentences in the options each contain proper punctuation or conjunctions that separate or connect ideas appropriately, thereby avoiding run-on structures.