C is your answer :)
Hope it helps!
The correct answer is by using the antagonist.
The antagonist is a molecule that blocks a biological response by binding to the receptor. So, you add antagonists to the receptors you want to determine and see which antagonist blocked the response. By blocking the specific response you can get the answer what receptor it was.
I think it will change, because over the years they have found different ways to show the atom, and different discoveries. We always have a new chance to discover something new about the atom.
The first, second, and fourth statements are correct.
