Since its a sibling, and siblings tend to be in a person's close circle of people, informal speech is used.
I think you could pick both B and D in this situation. You didn't include the passage so I have nothing to go on here except my little knowledge of the story from when I read it.
The correct answer is A. The author was confident in her preparation for the tryout.
Explanation:
In the paragraph, the narrator describes her perspective about the tryout, about this she specifies the routine was simple "The routine was easy to learn" and she had completely mastered the routine as the author explains " I practiced so much that I could do that routine in my sleep." In this context, it can be inferred or concluded she "was confident in her preparation for the tryout" because, in the text, most details focus on how easy the routine was and the abilities of the author to perform it.
