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). 

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