What form of reinforcement occurs when an ear pinch stops as the dog opens its mouth to take the dummy?

In the scenario described, the form of reinforcement at play is negative reinforcement. This occurs when a behavior is strengthened by the removal of an aversive stimulus. In this case, when the dog performs the desired action—opening its mouth to take the dummy—the ear pinch (an unpleasant sensation) is removed, leading to the cessation of discomfort.

The key factor here is that the removal of an unpleasant stimulus (the ear pinch) after the desired behavior (opening the mouth) encourages the dog to repeat that behavior in the future. This aligns perfectly with the principles of negative reinforcement, which focuses on increasing the likelihood of a behavior by removing negative conditions.

Other options, such as positive reinforcement and positive punishment, do not apply to this situation as they involve adding a stimulus rather than removing one. Extinction refers to the reduction of a behavior when reinforcement is no longer provided, which doesn't fit the context of stopping an unpleasant sensation.