to form the plural of a noun ending in  s, sh, ch,  or  x,  you should add                a.  s.      b.  ies.      c.  ves.      d.  es. 

D. -es

Basic grammatical rule. Best way to remember those is by reading it aloud and judging it by how familiar it sounds. Example: singular pass becomes passes, arch becomes arches, fax becomes faxes, and stash becomes stashes. Of course, however, there are exceptions, but I hope this helped nonetheless.