When given the false belief task described in the linked video, a child that does not yet have a fully-developed theory of mind would reply that sally would look for the ball in a child who has developed this ability would reply that sally would look in

The box;    the basket

Explanation:

Theory of Mind refers to the ability to comprehend other people's emotional mental states and acknowledge that mental states may differ from ours.

Developing a mind theory is a main phase in the development of a child.

A very well-developed mind theory helps individuals resolve conflicts, form relationships, and relatively predict the behavior of other people.