A is correct. B is wrong because when you say "is" then this means a singular so the ending has to be Story not stories. C is wrong because it's not follow but follows as stories are plural. D "Whag" is not a word and again can't use follow and is with stories

What follows next are their stories. (A) (Wat hierna/erna volgt, zijn hun verhalen) (In Dutch, there is a word ‘er’ that can be used both singular and plural) (‘What/wat hierna/erna' is singular and ‘stories’ is plural)

Excellent, excellent question! D feels completely wrong. When I asked 2 other American native speakers, both instinctively felt like A was the best choice, then realized that A seemed inconsistent and B was more logical, and finally realized that C could also work. Usually, verbs will be conjugated according to the subject, not the object. However, there are several places where the subject can be ambiguous. --Obviously, when the subject is "there": there is a book on the shelf// there are some books on the shelf. --When the subject contains "and": Peanut butter and jelly are two different condiments// peanut butter and jelly is my favorite kind of sandwich --America and Britain disagree on collective nouns: ----UK: My family is large (there are a lot of people in it)// my family *are* smart (each individual in my family is smart) ---US: My family is large// my family is smart (in America, collective nouns are always singular) Nominal clauses are weird. --Some seem like they should obviously be plural: "whichever books he read *were* helpful" --Some seem like they should obviously be singular: "No, she didn't go to Paris. Where she went *was* three cities in northern England." --and others seem almost totally ambiguous to me. "No, I don't have any bananas. What I have *is/are* oranges". "Everything is gone. All that is left *is/are* stories." --If you pushed me, I would say that singular is better here, but plural doesn't feel wrong to me . *** Obviously, the confusion goes away if we replace "what" with something more specific -- The thing that follows is (a collection of) their stories. -- The things that follow are their (separate and differing) stories. Since we usually interpret "what" as "the thing", we prefer "what follows" Since it is next to the plural "their stories", we prefer are. If we notice that this results in an inconsistent sentence, we change to B.