Final answer:
A sentence that reflects the tone of Helen's experience on her first night alone would communicate feelings of apprehension and self-doubt, corresponding to the actions of double-checking locks and windows, which indicate a lack of confidence.
Step-by-step explanation:
The sentence that consistently reflects the tone of the paragraph about Helen's first night alone in her new apartment is likely one that conveys uncertainty, apprehension, and a challenge to self-assurance. Since Helen is repeatedly checking the locks and making rounds to the windows, it shows a sense of unease and concern for her safety. This nervousness contradicts her earlier claim to her parents about her maturity and readiness to live alone. Therefore, a sentence that captures this mood could be, "Despite her earlier confidence, Helen felt a knot of anxiety tighten in her stomach with every click of a checked lock, the echoes of her parents' worried voices suddenly resonant in her mind."