Which observation indicates a client is experiencing an allergic reaction to a medication?

The observation indicating that a client is experiencing an allergic reaction to a medication is characterized by rashes and hives. Allergic reactions typically involve the immune system's response to a substance, which can lead to a variety of symptoms, with skin manifestations such as rashes and hives being among the most common. These skin reactions are often the body's way of signaling that it is reacting negatively to a medication, resulting from the release of histamines and other chemicals in response to the allergen. This direct correlation makes rashes and hives a distinctly indicative sign of an allergic reaction compared to the other listed options, which are less specific and can be associated with a variety of other medical conditions or responses.