Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of finite and nonfinite verb forms within a sentence. The nonfinite to-infinitive “to tell” is correctly used to form a nonfinite (infinitive) clause that explains the reason Engle uses poetry in her novel.
Why the other choices are wrong
Choice A
Choice A is incorrect because the finite present tense verb “tells” can’t be used in this way to explain the reason that Engle uses poetry in her novel.
Choice B
Choice B is incorrect because the finite past tense verb “told” can’t be used in this way to explain the reason that Engle uses poetry in her novel.
Choice C
Choice C is incorrect because the finite present progressive tense verb “is telling” can’t be used in this way to explain the reason that Engle uses poetry in her novel.