Collective Noun: herd

The collective noun 'herd' is used to describe a group of grazing animals like sheep and cattle. Gulls are a type of seabird, which is why it is the odd ones out.

Option A: Chamois are a type of goat-antelope that live in mountainous regions. Since they are grazing animals, they are correctly referred to as a herd.

Option C:  While walruses are marine mammals, they are often referred to as a herd when grouped together on land or ice. 

Option D: Wrens are small birds. The correct collective nouns for wrens include a "flock" or a "chime", not a "herd."