To determine which of the five people is the consistent truth teller, first ask A, "Are you the truth teller?" If the answer is yes, ask A, "Who is the truth teller?" If A points to himself, he is indeed the truth teller, because his answers are consistent. If he points to anyone else, then that person is the truth teller. (In this case, A¿s answers are inconsistent, so he must have lied the first time. Because A alternately lies and tells the truth, he will answer the second question truthfully.)
If A¿s answer to the first question is no, then A is telling the truth but is an alternating liar. So ask him, "Who is not the consistent truth teller?" His answer must be a lie, so he will point to the truth teller. This solution works for any number of people.