15. What is the most common way to form the plural of a noun? A. Add - ren B. Add - s C. Add - ves D. Add - es

Adding - s is the most common. for example: books, cups, pencils, etc. - es might have been the one that confused you, but that isn't a super common ending for plurals--instead, a more common one would be "-ies" which is when you drop the y (berry becomes berries, for example).