Which of these is not a response to internal stimuli?

eating when hungry

breathing hard when exercising because of a drop in oxygen levels

drinking when thirsty

wearing sunglasses to protect eyes from uv rays

Among the things you've provided - the option that doesn't resemble and also isn't a response to an internal stimuli would be wearing sunglasses to protect eyes from UV rays. In all the other cases, the person who does a certain action also gets a feedback from his body which urges him to do that certian action. This isn't the case in D where he sees the sun and knows about the bad UV rays and decides for that course of action.