Which example best illustrates a primary punisher?

A primary punisher refers to a stimulus that is inherently aversive and naturally leads to a decrease in the frequency of a behavior without the need for prior learning or conditioning. Exposure to extreme temperatures is a prime example of this because such stimuli typically create discomfort or pain directly associated with the biological necessity to avoid harmful temperatures. This discomfort leads to a natural avoidance behavior, making it an effective primary punisher.

In contrast, the other options involve learned behaviors or social contexts that may not produce immediate physical pain or discomfort. Scolding a child for misbehavior is a form of social punishment that relies on the child's learning about social norms. Receiving bad grades in school can be distressing, but it is a result of the social construct of grading which may vary in significance for different individuals. Being ostracized by peers involves complex social dynamics that can also depend on the individual's value placed on peer acceptance. None of these result in direct pain or aversive stimuli like extreme temperature does, which is why they are not classified as primary punishers.