Final answer:
The adaptation level theory describes how individuals adjust their expectations based on past experiences, influencing their perceptions and emotional responses to new stimuli.
Step-by-step explanation:
The phrase that best describes the adaptation level theory is 'a. Individuals adjust their expectations based on past experiences.' This theory falls under the broad category of psychological theories that explore how individuals perceive and respond to various stimuli based on their previous experiences and expectations.
Specifically, the adaptation level theory suggests that people continuously modify their expectations and standards for what is considered normal or acceptable based on their past experiences. This ongoing process of adaptive expectations influences not only how people react to changes in their environment, such as expected inflation or shifts in economic cycles (as per the neoclassical perspective), but also informs their emotional and subjective experiences.