Answer: C) That a river ran through the middle of it.
Step-by-step explanation: In the given excerpt we can see how the speaker, named Tommy Stubbins, tells his memories of when he was a kid in his home town Puddleby. In the description of the town, the element that is most significant to the plot of the story, is that a river ran through the middle of Puddleby, because Tommy emphasizes in how he used to sit on the river-wall to watch the sailors unloading the ships, and he also used to sing with them and dream about sailing.