Thermoregulation Question
You're right, A and B both are normal thermoregulatory responses, so those can't be the right answer.

I'm guessing the rationale for C is that, if the dog's body temperature is increasing to match that of the hot environment, it must mean the dog has an impaired thermoregulatory response. It's kind of tautological.

But D also seems correct to me. If a dog is in a hot environment, and its blood vessels near the skin are constricting (say, because it's sick), that's counterproductive to the normal thermoregulatory response. If the dog isn't dilating its blood vessels near the skin, that's an impairment to the normal thermoregulatory response.

Perhaps the question is written wrong? Maybe D was supposed to say "vasodilation" instead of "vasoconstriction"? If this is a question written by a teacher I'd say you have a case for arguing it.