A cat. Emma owns a cat, as indicated by the clues in the passage. Her pet "purrs" and has "whiskers", both of which are characteristics unique to felines. The other pets mentioned (dog, hamster) do not have whiskers or purr, so the only remaining option is that Emma has a cat. The text does not provide enough information to definitively determine the exact breed or name of the cat, but it is clear that Emma is the owner of this particular feline animal based on the description provided of her pet's purring and whiskers. So to summarize, Emma owns a cat, which can be stated simply as: Cat.