Final answer:
The correct object emphasized by Catherine Bush in her discussion about sustainability cannot be determined from the provided information. Consultation of the original text where Bush's views are discussed is necessary for an accurate answer.
Step-by-step explanation:
Without access to the specific discussion by Catherine Bush regarding sustainability through the lens of her fifteen-year-old niece, it is not possible to provide an accurate answer to which object she emphasizes as the focus of her thoughts.
The provided references to Lily Tran's work and various chapters about sustainability do not yield the necessary information to answer the question confidently. In such cases, it is always best to consult the original text or source material where Catherine Bush's discussion can be found to identify whether the focus is a tree, a smartphone, a book, or a bicycle.