The first thing that strikes me about this question is that actually the scholar has two hypotheses, one is that the date is incorrect and the second is that the reason the date is incorrect is that a mistake was made in using Roman numerals. So in going into the answer choices I was not 100% clear regarding what is to be supported.

(A) This makes essentially no difference. It doesn't say that Thorpe did die before 1460 or that there is reason to believe that he did. So neither of the scholar's hypotheses is supported by this.

(B) Naturally a preface to a book that attributes both works to Thorpe would attribute both works to Thorpe, whether that attribution is correct or incorrect. Meanwhile, the attribution of the works is irrelevant to a discussion of hypotheses about dates.

(C) While by making up some kind of connection one could construe this answer choice as being somehow relevant to the discussion of the dates, really it is not. We are not concerned with the practices of the times, only whether the dates were written correctly. Making up connections that are not really logically supported by what is said in parts of CR questions is a great way to get them wrong.

(D) While many years have the date, "Friday, September 20," in them, apparently 1460 did not. So if that date was contemporaneous with the writing of the Testament, then we have reason to believe that 1460 is not the year that the Testament was written. "Friday, September 20" did however occur during 1409. So that the Testament was written in 1409 seems more likely than that it was written in 1460, and thus this choice in a way supports the hypothesis that the correct year is 1409. Key to seeing that this is the correct answer is noticing that the prompt says that the date, "Friday, September 20," was contemporaneous with the writing of the Testament. Noticing key details is paramount for getting CR questions right.

(E) This is a nice trap answer in that it mentions a date, 1406, that is much closer to 1409 than to 1460. Notice, however, that 1406 is before both 1409 and 1460. Also, this says that "few" rather than none of the references occurred later than 1406, meaning that some occurred after 1406 and some could have occurred even after 1409. So this does not really support the idea that 1409 rather than 1460 is the correct date.

So the correct answer is D.