C. I thought about what he'd said soon I realized he was right.
A run-on sentence is a sentence that contains two independent clauses that are not joined with the correct punctuation. An independent clause has a subject and verb and contains a complete thought. Sentence C has two complete thoughts with a subject and verb, but no punctuation to connect them correctly. The independent clauses are "I thought about what he'd said" and "Soon I realized he was right". To correct this sentence the author would need to use a semicolon (;) between the clauses or use a comma and conjunction. While some of the other sentence contain extra conjunctions (option B and D), they don't have two subject and verb phrases. Option A uses a semicolon to join the two independent clauses which makes it grammatically correct.
