Final answer:
The behavior of the client with schizophrenia represents the defense mechanism called regression, where they revert to child-like behaviors in response to anxiety.
Step-by-step explanation:
The 23-year-old client with schizophrenia who curls up in a ball and sucks his thumb when his father visits is exhibiting the defense mechanism known as regression. Regression is an ego defense mechanism in which a person faced with anxiety reverts to behavior characteristic of an earlier stage of development. In this case, the client's actions resemble behaviors typical of a much younger child, which provides a temporary escape from the stressful situation.