C. The book's story was pretty exciting, even though its cover was not.
The apostrophe rule we are looking for here is the possessive rule for the apostrophe. Because only one book is being talked about, we’ll need the singular possessive form, which is book’s. The rule of adding an apostrophe has, however, an exception. For the pronoun “it,” an apostrophe “s” indicates the contraction of “it is.” As such, to make it possessive, only an “s” is added (without an apostrophe) as in its.