Final answer:
Pointing to a hidden object for someone else to find indicates that a child has developed Theory of Mind, as it shows understanding of others' perspectives.
Step-by-step explanation:
During the mean monkey task, a behavior that indicated one of the children had begun to develop Theory of Mind (ToM) is C) Pointing to a hidden object for someone else to find. This behavior demonstrates the child's understanding that others have their own thoughts and perceptions. Theory of Mind is a significant cognitive milestone where children realize that different people might have different beliefs, knowledge, and emotions. This ability allows children to predict or explain others' actions and to understand that others can hold incorrect beliefs.