What limitation of activities as reinforcers might lead to decreased effectiveness over time?

The correct answer is based on the concept of satiation, which refers to the condition in which an individual becomes too accustomed to a reinforcer, leading to a decrease in its effectiveness over time. When a specific activity is used repeatedly as a reinforcer, the individual may reach a point where they no longer find it enjoyable or motivating. For instance, if a child consistently receives praise or access to a favorite toy for good behavior, over time, the impact of that praise or toy may diminish because the child has experienced it too often. This is akin to how consuming the same food every day can lead to a lack of appetite for it since the novelty wears off.

In the context of behavioral reinforcement, it's crucial for practitioners to vary the reinforcers provided to maintain their potency and encourage desired behaviors effectively. Varied reinforcers help sustain enthusiasm and motivation, avoiding the pitfall of diminishing returns from any single invariable activity.