Explanation
Correct answer: A.
Choice A is the best answer because it most accurately describes how the underlined sentence functions in the text as a whole. In the text, the narrator makes observations about her young niece, Dot, noting that Dot is making progress toward outgrowing babyhood and is growing stronger "day by day." The description of Dot purposefully exercising by "bouncing for hours" reinforces the narrator’s earlier characterization of Dot. The underlined sentence thus offers a detail that shows how Dot is growing stronger.
Why the other choices are wrong
Choice B
Choice B is incorrect. While a sentence earlier in the text mentions Celestine’s psychological state (sadness because Dot is growing fast), the underlined sentence doesn’t focus on Celestine at all. Instead, the sentence focuses on Dot’s physical activities to show that she is getting stronger.
Choice C
Choice C is incorrect because the underlined sentence describes Dot’s behavior, not the narrator’s psychological characteristics. While a sentence earlier in the text establishes that the narrator, like Dot, was "impatient with babyhood," the underlined sentence specifically addresses Dot’s physical activities in her stroller.
Choice D
Choice D is incorrect. While an earlier sentence in the text mentions that Celestine is sad, the underlined sentence doesn’t focus on how Celestine feels nor does it describe a behavior that Celestine engages in. The sentence focuses entirely on Dot’s actions, describing her as "bouncing for hours" to show that she is growing stronger.