Final answer:
Rachel's thesis is most closely associated with sentence b, which mentions the inspiration from a teacher that sparked her interest in engineering and suggests the focus of her writing.
Step-by-step explanation:
Rachel's thesis is most closely stated in sentence b. This sentence not only informs us about when Rachel became interested in her future beyond high school, but more specifically pinpoints the inspirational moment and the area of interest: engineering. This declaration provides a clear direction for her writing, indicating that engineering has played a significant role in shaping her aspirations and potential future career path. It acts as a setup for her central argument or main point of discussion throughout her work. In contrast, the other sentences serve as background information, specific examples, or supporting details rather than the thesis itself.