Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is punctuation use between sentences. In this choice, the period is used correctly to mark the boundary between one sentence ("Philosopher...aesthetics") and another ("As...formalist"). The long adverbial element "as evidenced...itself" modifies the content in the following clause to support the claim that Kivy’s approach was formalist.
Why the other choices are wrong
Choice A
Choice A is incorrect because it results in a run-on sentence. The sentences ("Philosopher...aesthetics" and "As…formalist") are fused without punctuation and/or a conjunction.
Choice B
Choice B is incorrect. Without a comma preceding it, the conjunction "and" can’t be used in this way to join sentences.
Choice C
Choice C is incorrect because it results in a comma splice. A comma can’t be used in this way to mark the boundary between sentences.