The reference that lists available magazine articles alphabetically by subject is the encyclopedia newspaper index card catalog readers' guide

I would go with Readers' Guide, because it lists hundreds of magazines. Usually, there is a reference list for all the available magazine articles that is arranged alphabetically by subject. This is called a reader's guide. A newspaper index is a reference for newspapers base on the issue date while a card catalog is a reference in finding authors of books. So the correct answer is C. I hope this answer helped you <3

A. an is indefinite article