Answer: The sentence that uses 'who' correctly is "Who is the person that I should call?".
Explanation: 'Who' is a relative pronoun and, unlike 'which', it is used to refer to people. Moreover, it is used to refer to the subject of a sentence. However, it should not be confused with 'whom', which is another relative pronoun since 'whom' should be used when referring to the object of a verb or a preposition. In the sentence 'Who is the person that I should call?', the use of 'who' is correct because it makes reference to the subject of that sentence. In contrast, the use of 'who' in the other options is incorrect: 'whom' is the relative pronoun that should have been used to function as object of the verb and object of the prepositions 'from' and 'to'.
