Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a comma to mark the boundary between the main clause ("Recently...supposed") and the supplementary element ("a finding...Paleolithic") that provides additional information about the implications of the Neronian tool discovery.
Why the other choices are wrong
Choice A
Choice A is incorrect because a semicolon can’t be used in this way to join the main clause ("Recently...supposed") and the supplementary element ("a finding...Paleolithic").
Choice B
Choice B is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with "a finding."
Choice C
Choice C is incorrect because it results in a run-on sentence. The main clause ("Recently...supposed") and the supplementary element ("a finding...Paleolithic") are fused without punctuation and/or a conjunction.