The correct pronoun to fill in the blank is C, "she". Therefore, the sentence would be as follows: The best prepared guides, Riley and she, seldom have problems on hikes. The NP "Riley and she" is an apposition that further specifies the NP subject "the best prepared guides". It gives the names of the guides.
The pronoun should be in the nominative case because that is the English grammatical case used for a nouns and pronouns when they are the subject of the verb.