OG详解-OG9 文法 Q21

Choice D is the best answer. The convention being tested is the use of punctuation around noun phrases. No punctuation is needed because the noun phrase “aluminum oxide” is a restrictive appositive, meaning that it provides essential identifying information about the noun phrase before it, “the chemical compound,” and thus doesn’t require punctuation around it. 
Choice A is incorrect because no punctuation is needed. Choice B is incorrect because no punctuation is needed. Choice C is incorrect because the noun phrase “aluminum oxide” is a restrictive appositive. Setting the phrase off with punctuation suggests that it could be removed without affecting the coherence of the sentence, which isn’t the case.