ume you have two separate copper tubes of O.D. 5/8” and I.D. 5/16”?

O.D. and I.D. mean 'Outside Diameter" and "Inside Diameter". The rest of the question is broken into several parts, asking you to determine how much energy can be put through the pair of tubes by using them in various ways. Each calculation will be different, and you will need to identify the equations for how to calculate energy transfer in a certain way. It is also giving you hints on things you should not neglect, like energy losses, and what you may ume you have.