I saw a ...... of cows in the field.

Answer: Option B

Option A: group
While "group" is a general term for a collection of individuals, it is not specifically used to refer to cows. It can apply to various types of collections.

Option B: herd
This is the correct option. "Herd" specifically refers to a group of cows or other grazing animals.

Option C: swarm
"Swarm" typically refers to a large group of insects or small animals that move together, not cows.

Option D: flock
"Flock" is used for a group of birds, such as chickens or ducks, but not specifically for cows.

In conclusion, the correct answer is Option B: herd, as it is the appropriate term for a group of cows in the field.