Final answer:
Sally demonstrating 'baby talk' following her baby brother's arrival is an example of regression, a defense mechanism where an individual returns to earlier development behaviors due to stress.
Step-by-step explanation:
When Sally, who is 6 years old, begins to exhibit "baby talk" after the birth of her baby brother, this behavior is an example of regression. Regression is a defense mechanism where an individual reverts to an earlier stage of development in response to stress or conflict. In Sally's case, the arrival of a new sibling might have caused her to feel insecure or jealous, leading her to subconsciously return to behaviors characteristic of a younger child to receive attention and care.