Concept:Collective nouns are words used to name a group of animals, people, or things.Explanation:Among the given options, "herd" is the standard collective noun for cattle or cows.A "flock" is used for birds or sheep, a "skein" for geese in flight, and a "gang" for workers or criminals.Therefore, the sentence that refers to a group of cows should be completed with "herd".Answer:B. herd