The correct answer is C. To Americans and Canadians
Step-by-step explanation:
In grammar, a preposition phrase includes a preposition such as on, about, to, by, of, etc. link to a word or more words that are nouns or pronouns and act as the object of the preposition. Additionally, in sentence prepositional phrases are considered as units and act as objects, adjectives or adverbs. Considering this, in the case of the sentence "Frederick Douglass spoke to Americans and Canadians" the words that are a part of a prepositional phrase is "to Americans and Canadians" because this contains the preposition "to" and two nouns that act as objects of it and complete the idea.