Final answer:
The child is displaying the defense mechanism of regression.
Step-by-step explanation:
The defense mechanism displayed by the child in this scenario is regression. Regression is when an individual reverts to immature behavior or acts younger than their age in response to anxiety or stress. In this case, the child resumes bedwetting after the birth of a sibling as a way of seeking attention and regressing to a time when they were the center of their parents' attentio