(A) might slightly favor the descriptivists’ view. If communities left on their own develop very similar rules compared to other communities, that might constitute evidence that there are “natural” rules of language, which makes those rules more like scientific laws.

(B) provides corroborating evidence for the causal relationship. If it were true that people could be persuaded to use language in a particular way, we would expect the degree to which someone follows language rules to be correlated with how much they were persuaded to use those rules.

(B) helps to confirm that expectation. On average, stronger emphasis on grammar instruction in primary schools is more associated with sticking to the language rules taught in those schools. And, weaker emphasis on grammar instruction in primary schools is less associated with sticking to the language rules taught in those schools. That provides at least some evidence of a causal connection between degree of persuasion (as measured by the emphasis on grammar instruction) and following language rules.

(C) might slightly favor the descriptivists’ view. If both formal education systems and minimal education systems show a similar range of variation in how language is used, that suggests that language instruction (persuasion) might not have an impact on language use. There are, of course, some assumptions underlying this interpretation of (C). For example, we would have to assume that “extensive formal education systems” and “minimal education systems” indicate different levels of language instruction. That seems like a reasonable assumption. Even if it’s not, that would mean (C) simply wouldn’t have an affect on the author’s position.

(D) could be read to weaken the author’s position. If persuasion and training could influence language use, we’d expect that the students who get more prior instruction should agree on grammar rules than students who didn’t get much instruction. But (D) tells us the opposite is true.

(E) gives us evidence that might undermine the author’s defense. If certain communities have “large differences” between them, but have language practices that are “quite uniform,” that suggests the laws of language usage might be more natural and similar to scientific laws. After all, we might expect very different communities to have different language usage, since each community’s use of language would likely be influenced by different sources of persuasion.