The correct answer is C. A cannot be correct, as "who happen to be best friends" is an appositive/adjective, and not an adverbial phrase. B is also not correct as "on the numerous antique cars" is an object, as is "everyone who voted" in the last example. So, "considering ourselves to be antique car experts" is an adverbial phrase, as it you can ask the question "why" to get this answer, which means it is an adverb - it describes the verb "vote" in more detail.
