B) There are three ducks in the pond. -- Also possible.
C) There are five cows in the fence. -- No.
D) There are five cows inside the fence. -- OK if you specially want to emphasise the fence. However, you would normally say "in the field" or similar, and it would be assumed that some sort of fence or other barrier was in pla
1. When I am on a farm, can I say "there are a herd of sheep on the grass" when they're outside the fence? Or should I simply say "there are a herd of sheep in the field"?
2. What's the difference between "in the field" and "on the field"?
1. I prefer "There is a herd of sheep on the grass." This does not, by itself, say anything about the presence or absence of fences, or about where the sheep might be located in relation to the fences. It just means that there is a piece of grass with a group of sheep on it.
A "field" normally refers to an enclosed area of land, so "There is a herd of sheep in the field" implies