It is whom. You can only use who if you can replace is with he or she.
The answer is: C. subject pronoun
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Subject pronouns like <em>I, she, he it, you, we </em>and <em>they </em>replace a noun as the subject or main actor of a sentence or clause. They also function as the subject of the verb, so they perform the action of the predicate.
The rest of the alternatives are incorrect because possessive pronouns indicate possession (what belongs to whom), object pronouns work as direct or indirect object of the verb, and reference pronoun refers to pronoun agreement.
Answer:
Sandburg's use of literary techniques such as onomatopoeia, alliteration, repetition, and nonsense words makes the story playful and cheerful.
