Final answer:
Betsy's loss of enjoyment in painting after becoming a professional portrait painter is an example of the overjustification effect, where external rewards diminish intrinsic motivation. The correct answer is option: over justification effect.
Step-by-step explanation:
The change in Betsy's motivations after becoming a professional portrait painter is known as the overjustification effect. This is a psychological phenomenon wherein an external incentive like money decreases a person's intrinsic motivation to perform a task they previously enjoyed. In essence, when a hobby becomes a job and the person is compensated for it, the intrinsic pleasure once derived from the activity diminishes and the activity is now seen as work.
This effect occurs because the external reward may overshadow the internal joy and satisfaction that comes from the activity itself. Over time, this reliance on extrinsic rewards can lead to a decrease in the intrinsic motivation that initially drove the person to engage in the activity for its own sake.