A run on sentence is a sentence that runs on without pause that needs a pause, genes it being called run on. When reading English , the comma shows a pause in the sentence and allows a natural flow. From choices A - C the sentences have flow and include a pause due to the comma. Choice D does not include a comma and when read aloud it sounds rushed and "run on." If it were to be corrected, answer choice D would have a comma placed after the word "flowers".