Welcome to one of the easiest algebraic equations in the world!... or is it? MUAHAHAHAHA!
Can you solve this pesky little problem while also giving damning evidence that the other 2 can not possibly work?
YOU MUST CHOOSE ONE AND ONLY ONE!
ADDED: Those calculations are simply thought processes for each possible answer. I am aware that there may or may not be something missing that could help clarify, but I am leaving it up to you to figure it out.