Let’s examine the sentence: “I saw a _____ of cows in the field.” The sentence requires a group word for cows. In English language, different group words are used for different animals and things. So, “swarm” is used for the group of flying insects, “flock” is used for a group of sheep, goats, or birds and “group” is used more generally of a collection of any living and non-living thing. The group word used specifically for cows is “herd.” Hence, option B is the correct answer.