Zachary: The term "fresco" refers to paint that has been applied to wet plaster.
Zach's Conclusion (therefore)
In order to restore Michaelangelo's Sistine Chapel paintings to the appearance that Michelangelo intended them to have, everything except the original fresco work should be stripped away.
Zach's Evidence
Additions known to have been made by later painters have obscured the original fresco.
Stephen's Response
But it was super common back then for painters of a given fresco to add painted details to their own frescoes after the frescoes had dried.
Evaluate
Stephen is suggesting that Zach's plan might backfire. Zach is hoping to strip away all the layers other than the original fresco, so that it looks the way Michaelangelo intended.
But Stephen is saying,
Goal
In terms of the popular categories of Method answers (analogy, counter/example, bad implications of logic, ruling out alternatives, suggesting alternate explanation, raising overlooked consideration, calling out a dubious assumption), we would probably say that this one is Raising an Overlooked Consideration --
Or we could say it's implicitly revealing an assumption of Zach, that .
Look for anything we can match to the text.