Final answer:
The approach to understanding motivation based on the belief in the need for stimulation is the arousal theory approach.
Step-by-step explanation:
The approach to understanding motivation that is based solely on the belief in the need for stimulation is the arousal theory approach. This approach suggests that individuals are motivated to maintain an optimal level of arousal, seeking out stimulation when bored and engaging in behaviors to reduce arousal when overstimulated. According to this theory, simple tasks are best performed when arousal levels are relatively high, while complex tasks are performed best when arousal is lower.