What happens next? Your boss interrupts you and kindly asks you to do some bloody work!
Right, now I've probably given this too much thought (not often I can say that!!) and I say Hot Water.
The reasons? Simple - No-one with an ounce of sense would drink anything from a vending machine! I'm guessing you don't have access to a kettle at work so the clever thing to do would be to use the hot water and bring your own tea bags/coffee to work with you.
Furthermore you say you have a cold, so maybe you bought the hot water to make your self a lemsip drink?
Third reason: Plain hot water probably tastes better then the other options!