Answer:
It is a little bit trivial:
As we know:
Energy sink: is anything that uses energy.
Energy source: is anything that produces energy.
How the caol deposits can be energy sinks? well, you want to extract the coal from those deposits, and to do it, you need to "use energy" (the type of energy can depend on the extraction method, for example, if you use a pickaxe to extract the coal, you are using mechanical energy)
These coals are potential energy sources because coal is an energy source, potential means that, given some circumstances, something can transform into other things.
So potential energy source in this situation means that those coal deposits can be potential energy when we extract the coal and use it as an energy source.
