Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period after “walls” is used correctly to mark the boundary between the first sentence (“In...walls”) and the second sentence (“With…techniques”), which starts with a supplementary phrase.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences.
Choice B
Choice B is incorrect because it results in a run-on sentence. The sentences (“In...walls” and “with...paintings”) are fused without punctuation and/or a conjunction.
Choice C
Choice C is incorrect. Without a comma preceding it, the conjunction “so” can’t be used in this way to join sentences.