Explanation
Correct answer: D.
Choice D is the best answer. The conventions being tested are the use of possessive nouns and the use of possessive determiners. The singular possessive noun "phrase’s" correctly indicates that there is only one simple phrase. The singular possessive determiner "its" agrees in number with the singular possessive noun "phrase’s," reinforcing the idea that there is only one simple yet complex phrase.
Why the other choices are wrong
Choice A
Choice A is incorrect because the context requires the singular possessive noun "phrase’s," not the plural possessive noun "phrases’."
Choice B
Choice B is incorrect because the context requires the singular possessive noun "phrase’s" and the corresponding singular possessive determiner "its," not the plural noun "phrases" and the corresponding plural possessive determiner "their."
Choice C
Choice C is incorrect because the context requires the singular possessive determiner "its," not the plural possessive determiner "their."