This might be tempting. You might be thinking that if rock varnish itself contains some organic matter, then maybe the organic matter, while from some older source, accumulated on the monument after 1492. But this thinking doesn't work for two reasons: first, we're talking about the organic matter found underneath the rock varnish layer, not in the rock varnish layer. Second, we're taking it as a given (one of the premises) that the organic matter found underneath the rock varnish grew on the stones shortly after they were trimmed. So even if the rock varnish contains some organic matter, that doesn't do anything to explain the presence of the organic matter underneath the varnish, which grew on the stones after trimming — and so this answer choice doesn't weaken the argument.

Correct. This answer choice takes advantage of an important assumption in the argument. Notice that the argument about the organic matter makes a very strong case that the stones were trimmed over 1,000 years ago — but the stimulus then jumps to a conclusion about when the monument was built. In other words, the argument assumes that the monument was also built shortly after the stones were trimmed. But if this answer choice is true, then that assumption is undermined. Even if the stones themselves are ancient, that doesn't prove anything about the age of the monument itself, which could have been built after 1492.

Irrelevant. Knowing that the monument is similar to other "ancient" sites in other parts of the world doesn't do anything to this argument.

Irrelevant. Just because the earliest written reference is from after 1492 doesn't mean the monument is also from after 1492. There could have been earlier written records that have been lost, or perhaps the culture that built the monument didn't have writing.

Irrelevant. We don't know that this monument was found in a "dry, sheltered" place, so we don't know if this fact even applies to the monument. Even if it did, we know that rock varnish did develop on this monument, and that it trapped organic matter underneath it that was over 1,000 years old. This answer choice doesn't provide an alternative explanation of that fact to what the stimulus provides, so it doesn't weaken the argument.