Millie dropped her doll while she was playing with it and rocking it. It landed out of view behind the rocking chair where Millie was sitting. A few moments later, Millie climbed down and retrieved the doll. Millie's behavior suggests that she has acquired the understanding of:

Object permanence

Explanation:

Object permanence: In psychology, the term "object permanence" is described as one of the important milestones that are being achieved by an infant of age between four-seven months. Object permanence involves an infant's understanding that when specific things tend to disappear, they are not gone or disappear forever.

In case, an infant hasn't yet developed the understanding of object permanence, then the things that leave his or her view tends to be gone, or completely gone.

In the question above, Millie's behavior suggests that she has acquired the understanding of object permanence.

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