The correct option is C Herd
The options are all collective nouns. Collective nouns refer to a number of things of people as one.
Option B: 'Herd' is a collective noun used for cattle and other livestock animals. Cows are included in cattle. Hence B is the correct option.
Option A: A group refers to a collection of people and not cows.
Option C: A swarm is a collective noun for bees or locusts. It does not refer to cows.
Option D: 'Flock' is used for sheep or birds. It does not refer to cows.
Hence A, C and D are incorrect.