Giving a brief description of a book is the situation in which summarizing would be the most useful. Option C is correct.
To summarize a text consists on writing its essential concepts into a brief paragraph, or two. When summarizing, it is not necessary to provide great detail of the book, text or story. On the contrary, you should mention the overarching ideas, identifying the general concepts that run through the entire piece and to convey these ideas employing precise, specific language.