What do we call an environmental event that alters the value of a stimulus?

The correct answer is "Motivating Operation" because this term refers to any environmental event or condition that temporarily changes the value of a stimulus, thus influencing behavior. Motivating operations can either increase (establishing operations) or decrease (abolishing operations) the effectiveness of a reinforcer or punisher.

In the context of behavior analysis, the concept of motivating operations is crucial for understanding how different factors can affect an individual's behavior towards specific stimuli. For instance, if an individual is deprived of water, the value of water as a reinforcer increases significantly, motivating the individual to engage in behaviors that will lead to obtaining water.

The other choices provided do relate to similar concepts but do not encompass the entirety of what a motivating operation defines. Establishing operations specifically increase the effectiveness of a reinforcer, while abolishing operations decrease it. The idea of a "contingency operation" does not correspond to established terminology in behavior analysis. Therefore, recognizing the broad impact of motivating operations is key to understanding behavioral responses in various environmental contexts.