Why was the narrator asked to write about Gertrude's new book?

The choice indicating that the narrator's relationship with Gertrude grew strong is accurate as it suggests a personal connection that likely influenced the request for him to write about her new book. A close relationship implies trust and a deeper understanding of Gertrude's work, making the narrator a suitable candidate for providing insights into her new publication. Such dynamics often lead individuals to contribute to a friend's work more readily, as they can offer a unique perspective that others may not have.

In contrast, the other options suggest different reasons that could involve professional qualifications or formal roles, such as being an expert or her personal assistant. While these reasons can be valid in some contexts, they do not inherently encompass the emotional resonance and personal insight that a strong relationship would provide, which can significantly enhance the depth of the writing. Additionally, having previously published a book might position the narrator as knowledgeable, but it doesn't necessarily connect to the nature of his relationship with Gertrude. Therefore, the growing bond between the narrator and Gertrude is the most compelling reason for his involvement in writing about her new book.