The argument is that since rock varnish shows the stone was cut 1000 years ago, the monument it is in must be older than 500 years.
B directly weakens it by showing that just because the rock was cut at a certain time, it doesn't mean that the monument was made at the same time.
D does look directionally useful, but doesn't really tell us much of anything. It shows a lack of evidence, which we know is not evidence of a lack. If d told us a bit more, such as that most other monuments were described in writing, this might weaken, but as it stands d just doesn't go far enough.
