Five-year-olds who were surprised to discover that a Band-Aids box contained pencils subsequently delighted in anticipating their friend's false belief about the contents of the box. This best illustrates that the children had developed a(n):

Theory of the mind

Explanation:

Theory of mind is the ability for one to  attribute desires, intents, beliefs, knowledge, emotions, etc. to others and to one's self,  and to understand that others have their own beliefs, intents, and perspectives that can most likely be different from one's own. In other words, an individual understands others have a mind of their own.

Theory of mind is important in social interactions and it is useful in judging and analyzing the behaviors of others.

In the case of the five year olds, they were able to accept each others idea of what is contained in the band aid box, because they had developed the theory of the mind.

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