Final answer:
The repeated element that builds tension in the story is the different sounds of laughter, contrasted with moments of distress or seriousness. Vivid details and imagery, such as those employed by Twain, further enhance this tension.
Step-by-step explanation:
In the context of the provided excerpts, the element repeated so that tension builds in the story could be interpreted as the different sounds of laughter. The varied descriptions allude to tension or unease juxtaposed with laughter or merriment—whether it be with the boughs shaking like a girl's school in the sun or the laughter of daffodils that seem to mock the characters amid their distress. The repetition of laughter in these scenes builds tension as it contrasts with the characters' inner turmoil or the gravity of their situation.
Vivid details and imagery also play a crucial role in creating tension, as seen in the sensory experiences of the traveler in the forest, where the soundscape amplifies his fear. Twain's use of such vivid depictions serves to immerse the reader and build anticipatory anxiety.