Why is the following statement untrue If (a) my aunt lies most of the time and (b) she says the FBI is tapping her phone calls then (c) the FBI is not tapping her phone calls.?

The statement is untrue because the conclusion that "the FBI is not tapping her phone calls" does not logically follow from the premises. Just because your aunt lies most of the time does not mean that everything she says is false; she could be telling the truth in this instance. Therefore, it’s possible that the FBI is indeed tapping her phone calls, despite her general tendency to lie.