Final answer:
The sentence that best supports toddlers experiencing rapid emotional changes is the one explaining the three-step process of emotional contagion.
Step-by-step explanation:
The sentence from the passage that best supports the answer in part a, which describes toddlers cycling through emotions quickly, is: "Emotional contagion is 'a three-step process through which one person's feelings transfer to another person' (Lewandowski, 2)." This sentence supports the concept that emotions can change rapidly and be influenced by others, similar to how a toddler's mood can shift suddenly from being extremely happy to extremely sad. The process of emotional contagion could contribute to these quick transitions in emotional states, as observed in the figure presented.