Explanation
Correct answer: C.
Choice C is the best answer. The convention being tested is the use of verbs to express tense in a sentence. In this choice, the future tense verb “will learn,” used in conjunction with the phrase “in a later chapter,” correctly indicates that “you” (the reader) are going to learn about obsidian’s structure at some point in the future.
Why the other choices are wrong
Choice A
Choice A is incorrect because the past perfect verb “had learned” doesn’t indicate that the subject is going to learn about obsidian’s structure in the future.
Choice B
Choice B is incorrect because the past perfect progressive verb “had been learning” doesn’t indicate that the subject is going to learn about obsidian’s structure in the future.
Choice D
Choice D is incorrect because the present perfect verb “have learned” doesn’t indicate that the subject is going to learn about obsidian’s structure in the future.