Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The nonfinite present participle "including" is correctly used to form a supplementary element that interrupts the main clause "These extensive changes…reflected the poet’s evolving literary perspective and experience of the US Civil War." This supplementary element, offset by commas after "changes" and "material," provides examples of the changes Whitman made to Leaves of Grass.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in an ungrammatical sentence. The finite modal verb "would include" can’t be used in this way to form a supplementary element within the main clause.
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The finite past tense verb "included" can’t be used in this way to form a supplementary element within the main clause.
Choice D
Choice D is incorrect because it results in an ungrammatical sentence. The finite present tense verb "include" can’t be used in this way to form a supplementary element within the main clause.