To fill in the blank, the user needs to know the appropriate collective noun for a group of cows.
Option A, "group" is a collective noun that can be used to refer to a collection of people or things. However, it is not the correct term for a collection of cows.
Option B, "herd" is the correct collective noun for a group of cows. Cows are domesticated animals that are often kept together in a group, or herd.
Option C, "flock" is typically used to refer to a group of birds, such as a flock of ducks or geese.
Option D, "swarm" is used to describe a large group of insects, such as a swarm of bees or ants.
Therefore, the correct answer is:
The Answer is: B. Herd