How to Chose a Rocket Engine and Propellant

You can find rocket engines using ethanol, kerosene, hydrogen, xenon and now recently methane. What decides the propellant used?

Okay, so this probably isn’t a regular predicament for you, but perhaps when reading about various rocket companies like SpaceX, ULA, Arianespace you will see talk of different kinds of rocket engines and fuels like RP-1 (kerosene), hydrogen, methane etc. I will try to answer why a particular engine and fuel is typically chosen.