The store gave out samples by the dozen. - Is the correct phrase, as an adverbial phrase is a word/ phrase that modifies the/ qualifies an adjective. So the 'dozen' modifies the sentence.
D would turn out to be your answer
is the way the author expresses his attitude through his or her writing
Numerous international human rights documents firmly establish the principle that no human being can be “illegal” or outside the protection of the law. Yet despite the clearly established principle that discrimination and abuse based on immigration status are violations of human rights, U.S. government policies continue to sanction human rights violations against migrants and immigrants.
