The phrase isn’t a comment on the level of intellectual commitment. It’s on the level of support for the descriptivists’ view.

The phrase isn’t a comment on the idea of theory vs. observation. It’s a comment on the level of support for the descriptivists’ view. By calling the view an “article of faith,” that means the descriptivists hold their belief in the absence of evidence. The author isn’t saying that the descriptivists have a theoretical justification for their view. Although it’s possible that they have a theoretical justification, the author hasn’t affirmed that they do.

Another reason (B) doesn’t fit is that the phrase “article of faith” is a reference to a specific argument made by the descriptivists – that apparent changes in language usage that correlate with attempts to encourage those changes would have occurred even without that encouragement. (B)’s reference to “describing language usage” is too broad. The author isn’t commenting on descriptivists’ general approach to describing how people use language.

(C) is too general for the meaning of the phrase. The phrase “article of faith” is about a specific argument – the argument that examples of people changing language usage in apparent response to attempts to change usage involve changes that would have happened naturally. The author is saying that specific argument has no support. But the author is not saying that descriptivists hold on to their “views on language usage” (plural).

In addition, (C) states that the descriptivists hold onto their views despite “overwhelming evidence” against them. But the author’s point in the line we’re being asked about is that the descriptivists don’t have support for their own view. That doesn’t mean there exists evidence against the descriptivists' view.

(D) matches the meaning of the phrase. The author claims that the descriptivists don’t have any support for the claim that the changes in language we observed would have happened even in the absence of attempts to encourage those changes. The descriptivists don’t have any evidence, and simply hold their belief despite that lack of evidence.

The author does not suggest that any evidence cited by descriptivists has been discredited. The descriptivists, as reported by the passage, never said, “We have empirical evidence that language has never changed in response to persuasion.”