The general rule of thumb with informality is that you use it with people you know well. You probably don't know your boss's boss, and even then, respect is still expected, or your congressional representative, and it's even in the description with your older relative.
Most people would agree that you probably know your best friend pretty well, so they will be the best audience for an informal style.
All I know is that is not B (because I got it wrong in the test), and I don't think it's A, so it might be C or D. Hope this helps at least a little bit.
