What condition is characterized by hives in an animal?

Hives, also known as urticaria, are raised, itchy welts that can appear on an animal's skin due to an allergic reaction. When an animal exhibits hives, it signifies that there is an inflammatory response occurring, often as a result of exposure to an allergen, such as certain foods, insect stings, or medications. This condition is characterized by the rapid appearance of bumps or welts that may vary in size and can be very itchy for the animal.

In the context of the other conditions mentioned, nasal discharge usually indicates respiratory issues or infections, diarrhea often points to gastrointestinal disturbances, and tremoring could suggest neurological concerns or pain. While all these symptoms indicate problems, none of them directly relate to the specific response that manifests as hives. Therefore, the presence of hives is distinctively identified as a specific allergic reaction and is the correct answer in this context.