Answer:
Option-(C): The resource which is the most useful and distinguishing factor in order to explain the use of "their,there, and they're" is the usage guide, as it would tell us more about the proper use of the words in a proper way.
Explanation:
We all have face the same problem in some way to deal with the words like their,there, they're but there is always a way if there is a hill and for that reason any person who is concerned about the grammatically correct English we have the usage guide to tell us more about each factor. The usage guide is the manual for guiding any person when ever there are some issues with any of the words or phrases inside English language.
