Hi rjhyman!
Thanks for asking! In order to answer your question, let's start by discerning what the question is asking us to weaken, then go through the answer choices one by one.
The question stem asks us to weaken the contention that Venetian artists "had no practical experiences of the large-scale representation of familiar religious stories" contained in lines 40-42. Looking a bit above and below those lines, we see the additional context that Tuscan churches had large frescoes, whilst frescoes were virtually nonexistent in Venice because of the climate.
With this in mind, we can dig into the answer choices. Remember, we're looking for the answer choices that weakens the argument, so we can eliminate anything that strengthens or does nothing to the argument.
Answer choice (A) can be eliminated because it is irrelevant/out of scope. Does whether or not the style of secular historical paintings in a palace is similar to the style of Venetian narrative paintings impact whether or not Venetians had no practical experience with large religious narrative paintings? Nope!
Answer choice (B) can be eliminated for the same reason.
Answer choice (C) is the correct answer because it directly attacks the conclusion that Venetian artists "had NO practical experience" with the large paintings. Well, if Venetian artists were apprentices in Tuscany, where we know large frescoes were being painted, then that severely undermines that claim that Venetian artists did not have practical experience with large religious paintings because it provides a scenario in which they did get practical experience!
Answer choice (D) can be eliminated for the same reasons as (A) and (B) - it's irrelevant.
Answer choice (E) can also be eliminated for the same reason!
I hope this helps
Kate