The correct answer is D. Oxen's
Step-by-step explanation:
According to grammar, the plural form of the word ox that refers to a type of bull is "oxen" and therefore this should be the form used in the sentence. Additionally, in the English language, it is common to use the apostrophe (') along with "s" to express belonging or relationship, this applies to singular and plural nouns that do not end in "s" as those ending in "s" do no need in most cases another "s". This implies in the sentence "The _____ yoke was smooth and polished from long use" in which the possessive form of oxen should be used the correct word is "oxen's" as this includes the correct plural form and also the correct way of making the word possessive (apostrophe followed by an "s").