All of the provided responses are actions that can be considered responses to external stimuli. However, if you are looking for a short explanation or emphasis on the best fit, "wearing sunglasses to protect eyes from UV rays" and "breathing hard when exercising because of a drop in oxygen levels" are particularly direct responses to environmental stimuli (UV rays and low oxygen levels, respectively).
If I were to select one that stands out as a clear and specific external stimulus response, it would be "breathing hard when exercising because of a drop in oxygen levels." This action is a physiological response to the immediate environmental condition of exercising, directly linked to a stimulus affecting the body's need for oxygen.