Answer: The sentence that uses "who" correctly is A. "Who is the person that I should call?".
Explanation: "Who" is a relative pronoun that is used to replace a noun. Moreover, "who" should always be used to refer to the subject of the sentence. Sentence A. uses this pronoun correctly because it is functioning as the subject ("who" can be replaced with "she" or "he"). In contrast, in sentences B. and C., the pronoun is acting as object of the preposition and not as the subject; therefore, "who" has not been used correctly. In these two sentences, "who" should be replaced with "whom".