The phrasing and context of your question are a bit unclear, but it seems you are asking which of the provided options best describes a specific aspect of a book revision related to Harriet Tubman.
Without additional context, it's hard to definitively choose the right answer, but if you are looking for a choice that explains the rationale behind writing about Harriet Tubman, option d ("it explains why Bradford chose to write about Harriet Tubman in the first place") may be the most relevant.
If you could provide more details about the book or the revision being discussed, I could help refine the answer further.