Yes, sentence C, "Most people eat cereal, it's healthy," is an example of a run-on sentence. A run-on sentence occurs when two or more independent clauses are joined together without proper punctuation or conjunctions. In this case, both "Most people eat cereal" and "it's healthy" could each be independent sentences on their own, but they are not separated properly. One way to fix this run-on sentence would be to add a coordinating conjunction like "and" or "but" between the two clauses, or to separate them into two separate sentences.
