The correct answer is: B. the summons
A summons is a legal document that orders someone to appear in court. It is usually served by a sheriff or other law enforcement officer. In the sentence “Oliver was assigned the task of serving a summons on Mr. Higgins to appear in a court of law,” the word “summons” is a noun that refers to the legal document that Oliver was tasked with serving.
The other options are incorrect because they are not nouns. Option A, “a summon,” is a verb that means “to order someone to appear in court.” Option C, “the summon,” is a noun that is the plural form of “summon.” Option D, “No improvement,” is incorrect because the sentence is grammatically correct and does not need to be improved.