The correct answer is D. around.
A fence is a structure that is built to enclose an area. It can be made of wood, metal, or other materials. Fences are often used to keep people or animals out of an area, or to keep them in.
In the sentence “There is a fence around the pool,” the fence is located around the pool. This means that the fence is on all sides of the pool. The fence is not located on top of the pool, at the pool, or in the pool.
The other options are incorrect because they do not accurately describe the location of the fence. Option A, “on,” is incorrect because the fence is not located on top of the pool. Option B, “at,” is incorrect because the fence is not located at the pool. Option C, “in,” is incorrect because the fence is not located in the pool.