Final answer:
The situation of a lottery winner not being happier a year after their win is best explained by the a. adaptation level theory, where individuals return to a baseline happiness level over time.
Step-by-step explanation:
The situation described fits best with the a. adaptation level theory, which suggests that individuals have a baseline or reference point of happiness which they eventually return to after experiencing significant positive or negative events.
According to this theory, a person adapts to their new circumstances, such as winning a lottery, and their level of happiness will tend to regress to the baseline over time.
This phenomenon is supported by studies conducted by researchers like Brickman et al. (1978), which found that lottery winners were not significantly happier than before their win, after some time had passed.